| Message ID | 20241214013600.it.020-kees@kernel.org (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | [v2] fortify: Hide run-time copy size from value range tracking | expand |
On Fri, Dec 13, 2024 at 05:36:10PM -0800, Kees Cook wrote: > GCC performs value range tracking for variables as a way to provide better > diagnostics. One place this is regularly seen is with warnings associated > with bounds-checking, e.g. -Wstringop-overflow, -Wstringop-overread, > -Warray-bounds, etc. In order to keep the signal-to-noise ratio high, > warnings aren't emitted when a value range spans the entire value range > representable by a given variable. For example: > > unsigned int len; > char dst[8]; > ... > memcpy(dst, src, len); > > If len's value is unknown, it has the full "unsigned int" range of [0, > UINT_MAX], and bounds checks against memcpy() will be ignored. However, > when a code path has been able to narrow the range: > > if (len > 16) > return; > memcpy(dst, src, len); > > Then a range will be updated for the execution path. Above, len is now > [0, 16], so we might see a -Wstringop-overflow warning like: > > error: '__builtin_memcpy' writing between 9 and 16 bytes from to region of size 8 [-Werror=stringop-overflow] > > When building with CONFIG_FORTIFY_SOURCE, the run-time bounds checking > can appear to narrow value ranges for lengths for memcpy(), depending on > how the compile constructs the execution paths during optimization > passes, due to the checks on the size. For example: > > if (p_size_field != SIZE_MAX && > p_size != p_size_field && p_size_field < size) > > As intentionally designed, these checks only affect the kernel warnings > emitted at run-time and do not block the potentially overflowing memcpy(), > so GCC thinks it needs to produce a warning about the resulting value > range that might be reaching the memcpy(). > > We have seen this manifest a few times now, with the most recent being > with cpumasks: > > In function ‘bitmap_copy’, > inlined from ‘cpumask_copy’ at ./include/linux/cpumask.h:839:2, > inlined from ‘__padata_set_cpumasks’ at kernel/padata.c:730:2: > ./include/linux/fortify-string.h:114:33: error: ‘__builtin_memcpy’ reading between 257 and 536870904 bytes from a region of size 256 [-Werror=stringop-overread] > 114 | #define __underlying_memcpy __builtin_memcpy > | ^ > ./include/linux/fortify-string.h:633:9: note: in expansion of macro ‘__underlying_memcpy’ > 633 | __underlying_##op(p, q, __fortify_size); \ > | ^~~~~~~~~~~~~ > ./include/linux/fortify-string.h:678:26: note: in expansion of macro ‘__fortify_memcpy_chk’ > 678 | #define memcpy(p, q, s) __fortify_memcpy_chk(p, q, s, \ > | ^~~~~~~~~~~~~~~~~~~~ > ./include/linux/bitmap.h:259:17: note: in expansion of macro ‘memcpy’ > 259 | memcpy(dst, src, len); > | ^~~~~~ > kernel/padata.c: In function ‘__padata_set_cpumasks’: > kernel/padata.c:713:48: note: source object ‘pcpumask’ of size [0, 256] > 713 | cpumask_var_t pcpumask, > | ~~~~~~~~~~~~~~^~~~~~~~ > > This warning is _not_ emitted when CONFIG_FORTIFY_SOURCE is disabled, > and with the recent -fdiagnostics-details we can confirm the origin of > the warning is due to the FORTIFY range checking: > > ../include/linux/bitmap.h:259:17: note: in expansion of macro 'memcpy' > 259 | memcpy(dst, src, len); > | ^~~~~~ > '__padata_set_cpumasks': events 1-2 > ../include/linux/fortify-string.h:613:36: > 612 | if (p_size_field != SIZE_MAX && > | ~~~~~~~~~~~~~~~~~~~~~~~~~~~ > 613 | p_size != p_size_field && p_size_field < size) > | ~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~ > | | > | (1) when the condition is evaluated to false > | (2) when the condition is evaluated to true > '__padata_set_cpumasks': event 3 > 114 | #define __underlying_memcpy __builtin_memcpy > | ^ > | | > | (3) out of array bounds here > > Note that this warning started appearing since bitmap functions were > recently marked __always_inline in commit ed8cd2b3bd9f ("bitmap: Switch > from inline to __always_inline"), which allowed GCC to gain visibility > into the variables as they passed through the FORTIFY implementation. > > In order to silence this false positive but keep deterministic > compile-time warnings intact, hide the length variable from GCC with > OPTIMIZE_HIDE_VAR() before calling the builtin memcpy. > > Additionally add a comment about why all the macro args have copies with > const storage. > > Reported-by: "Thomas Weißschuh" <linux@weissschuh.net> > Closes: https://lore.kernel.org/all/db7190c8-d17f-4a0d-bc2f-5903c79f36c2@t-8ch.de/ > Reported-by: Nilay Shroff <nilay@linux.ibm.com> > Closes: https://lore.kernel.org/all/20241112124127.1666300-1-nilay@linux.ibm.com/ > Acked-by: Yury Norov <yury.norov@gmail.com> > Signed-off-by: Kees Cook <kees@kernel.org> > --- Fixed the build issues I have here, so: Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
On 12/14/24 07:06, Kees Cook wrote: > GCC performs value range tracking for variables as a way to provide better > diagnostics. One place this is regularly seen is with warnings associated > with bounds-checking, e.g. -Wstringop-overflow, -Wstringop-overread, > -Warray-bounds, etc. In order to keep the signal-to-noise ratio high, > warnings aren't emitted when a value range spans the entire value range > representable by a given variable. For example: > > unsigned int len; > char dst[8]; > ... > memcpy(dst, src, len); > > If len's value is unknown, it has the full "unsigned int" range of [0, > UINT_MAX], and bounds checks against memcpy() will be ignored. However, > when a code path has been able to narrow the range: > > if (len > 16) > return; > memcpy(dst, src, len); > > Then a range will be updated for the execution path. Above, len is now > [0, 16], so we might see a -Wstringop-overflow warning like: > > error: '__builtin_memcpy' writing between 9 and 16 bytes from to region of size 8 [-Werror=stringop-overflow] > > When building with CONFIG_FORTIFY_SOURCE, the run-time bounds checking > can appear to narrow value ranges for lengths for memcpy(), depending on > how the compile constructs the execution paths during optimization > passes, due to the checks on the size. For example: > > if (p_size_field != SIZE_MAX && > p_size != p_size_field && p_size_field < size) > > As intentionally designed, these checks only affect the kernel warnings > emitted at run-time and do not block the potentially overflowing memcpy(), > so GCC thinks it needs to produce a warning about the resulting value > range that might be reaching the memcpy(). > > We have seen this manifest a few times now, with the most recent being > with cpumasks: > > In function ‘bitmap_copy’, > inlined from ‘cpumask_copy’ at ./include/linux/cpumask.h:839:2, > inlined from ‘__padata_set_cpumasks’ at kernel/padata.c:730:2: > ./include/linux/fortify-string.h:114:33: error: ‘__builtin_memcpy’ reading between 257 and 536870904 bytes from a region of size 256 [-Werror=stringop-overread] > 114 | #define __underlying_memcpy __builtin_memcpy > | ^ > ./include/linux/fortify-string.h:633:9: note: in expansion of macro ‘__underlying_memcpy’ > 633 | __underlying_##op(p, q, __fortify_size); \ > | ^~~~~~~~~~~~~ > ./include/linux/fortify-string.h:678:26: note: in expansion of macro ‘__fortify_memcpy_chk’ > 678 | #define memcpy(p, q, s) __fortify_memcpy_chk(p, q, s, \ > | ^~~~~~~~~~~~~~~~~~~~ > ./include/linux/bitmap.h:259:17: note: in expansion of macro ‘memcpy’ > 259 | memcpy(dst, src, len); > | ^~~~~~ > kernel/padata.c: In function ‘__padata_set_cpumasks’: > kernel/padata.c:713:48: note: source object ‘pcpumask’ of size [0, 256] > 713 | cpumask_var_t pcpumask, > | ~~~~~~~~~~~~~~^~~~~~~~ > > This warning is _not_ emitted when CONFIG_FORTIFY_SOURCE is disabled, > and with the recent -fdiagnostics-details we can confirm the origin of > the warning is due to the FORTIFY range checking: > > ../include/linux/bitmap.h:259:17: note: in expansion of macro 'memcpy' > 259 | memcpy(dst, src, len); > | ^~~~~~ > '__padata_set_cpumasks': events 1-2 > ../include/linux/fortify-string.h:613:36: > 612 | if (p_size_field != SIZE_MAX && > | ~~~~~~~~~~~~~~~~~~~~~~~~~~~ > 613 | p_size != p_size_field && p_size_field < size) > | ~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~ > | | > | (1) when the condition is evaluated to false > | (2) when the condition is evaluated to true > '__padata_set_cpumasks': event 3 > 114 | #define __underlying_memcpy __builtin_memcpy > | ^ > | | > | (3) out of array bounds here > > Note that this warning started appearing since bitmap functions were > recently marked __always_inline in commit ed8cd2b3bd9f ("bitmap: Switch > from inline to __always_inline"), which allowed GCC to gain visibility > into the variables as they passed through the FORTIFY implementation. > > In order to silence this false positive but keep deterministic > compile-time warnings intact, hide the length variable from GCC with > OPTIMIZE_HIDE_VAR() before calling the builtin memcpy. > > Additionally add a comment about why all the macro args have copies with > const storage. > > Reported-by: "Thomas Weißschuh" <linux@weissschuh.net> > Closes: https://lore.kernel.org/all/db7190c8-d17f-4a0d-bc2f-5903c79f36c2@t-8ch.de/ > Reported-by: Nilay Shroff <nilay@linux.ibm.com> > Closes: https://lore.kernel.org/all/20241112124127.1666300-1-nilay@linux.ibm.com/ > Acked-by: Yury Norov <yury.norov@gmail.com> > Signed-off-by: Kees Cook <kees@kernel.org> > --- > Cc: Nathan Chancellor <nathan@kernel.org> > Cc: "Qing Zhao" <qing.zhao@oracle.com> > Cc: linux-hardening@vger.kernel.org > > v2: Make sure the expression statement ends with a single statement > v1: https://lore.kernel.org/all/20241213020929.work.498-kees@kernel.org/ > --- > include/linux/fortify-string.h | 14 +++++++++++++- > 1 file changed, 13 insertions(+), 1 deletion(-) > > diff --git a/include/linux/fortify-string.h b/include/linux/fortify-string.h > index 0d99bf11d260..1eef0119671c 100644 > --- a/include/linux/fortify-string.h > +++ b/include/linux/fortify-string.h > @@ -616,6 +616,12 @@ __FORTIFY_INLINE bool fortify_memcpy_chk(__kernel_size_t size, > return false; > } > > +/* > + * To work around what seems to be an optimizer bug, the macro arguments > + * need to have const copies or the values end up changed by the time they > + * reach fortify_warn_once(). See commit 6f7630b1b5bc ("fortify: Capture > + * __bos() results in const temp vars") for more details. > + */ > #define __fortify_memcpy_chk(p, q, size, p_size, q_size, \ > p_size_field, q_size_field, op) ({ \ > const size_t __fortify_size = (size_t)(size); \ > @@ -623,6 +629,8 @@ __FORTIFY_INLINE bool fortify_memcpy_chk(__kernel_size_t size, > const size_t __q_size = (q_size); \ > const size_t __p_size_field = (p_size_field); \ > const size_t __q_size_field = (q_size_field); \ > + /* Keep a mutable version of the size for the final copy. */ \ > + size_t __copy_size = __fortify_size; \ > fortify_warn_once(fortify_memcpy_chk(__fortify_size, __p_size, \ > __q_size, __p_size_field, \ > __q_size_field, FORTIFY_FUNC_ ##op), \ > @@ -630,7 +638,11 @@ __FORTIFY_INLINE bool fortify_memcpy_chk(__kernel_size_t size, > __fortify_size, \ > "field \"" #p "\" at " FILE_LINE, \ > __p_size_field); \ > - __underlying_##op(p, q, __fortify_size); \ > + /* Hide only the run-time size from value range tracking to */ \ > + /* silence compile-time false positive bounds warnings. */ \ > + if (!__builtin_constant_p(__fortify_size)) \ > + OPTIMIZER_HIDE_VAR(__copy_size); \ > + __underlying_##op(p, q, __copy_size); \ > }) > > /*This patch works for me. I tested it on PowerPC and x86-64 using GCC 13.X, CONFIG_FORTIFY_SOURCE=Y and CONFIG_NR_CPUS=2048. So, Tested-By: nilay@linux.ibm.com
From: Kees Cook > Sent: 14 December 2024 01:36 ... > In order to silence this false positive but keep deterministic > compile-time warnings intact, hide the length variable from GCC with > OPTIMIZE_HIDE_VAR() before calling the builtin memcpy. ... > diff --git a/include/linux/fortify-string.h b/include/linux/fortify-string.h > index 0d99bf11d260..1eef0119671c 100644 > --- a/include/linux/fortify-string.h > +++ b/include/linux/fortify-string.h > @@ -616,6 +616,12 @@ __FORTIFY_INLINE bool fortify_memcpy_chk(__kernel_size_t size, > return false; > } > > +/* > + * To work around what seems to be an optimizer bug, the macro arguments > + * need to have const copies or the values end up changed by the time they > + * reach fortify_warn_once(). See commit 6f7630b1b5bc ("fortify: Capture > + * __bos() results in const temp vars") for more details. > + */ > #define __fortify_memcpy_chk(p, q, size, p_size, q_size, \ > p_size_field, q_size_field, op) ({ \ > const size_t __fortify_size = (size_t)(size); \ > @@ -623,6 +629,8 @@ __FORTIFY_INLINE bool fortify_memcpy_chk(__kernel_size_t size, > const size_t __q_size = (q_size); \ > const size_t __p_size_field = (p_size_field); \ > const size_t __q_size_field = (q_size_field); \ > + /* Keep a mutable version of the size for the final copy. */ \ > + size_t __copy_size = __fortify_size; \ > fortify_warn_once(fortify_memcpy_chk(__fortify_size, __p_size, \ > __q_size, __p_size_field, \ > __q_size_field, FORTIFY_FUNC_ ##op), \ > @@ -630,7 +638,11 @@ __FORTIFY_INLINE bool fortify_memcpy_chk(__kernel_size_t size, > __fortify_size, \ > "field \"" #p "\" at " FILE_LINE, \ > __p_size_field); \ > - __underlying_##op(p, q, __fortify_size); \ > + /* Hide only the run-time size from value range tracking to */ \ > + /* silence compile-time false positive bounds warnings. */ \ > + if (!__builtin_constant_p(__fortify_size)) \ > + OPTIMIZER_HIDE_VAR(__copy_size); \ I think you can make that: if (!__builtin_constant_p(__copy_size)) \ OPTIMISER_HIDE_VAR(__copy_size) \ which is probably more readable. David > + __underlying_##op(p, q, __copy_size); \ > }) > > /* > -- > 2.34.1 > - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
On Sun, Dec 15, 2024 at 07:06:12PM +0000, David Laight wrote: > From: Kees Cook > > Sent: 14 December 2024 01:36 > ... > > In order to silence this false positive but keep deterministic > > compile-time warnings intact, hide the length variable from GCC with > > OPTIMIZE_HIDE_VAR() before calling the builtin memcpy. > ... > > diff --git a/include/linux/fortify-string.h b/include/linux/fortify-string.h > > index 0d99bf11d260..1eef0119671c 100644 > > --- a/include/linux/fortify-string.h > > +++ b/include/linux/fortify-string.h > > @@ -616,6 +616,12 @@ __FORTIFY_INLINE bool fortify_memcpy_chk(__kernel_size_t size, > > return false; > > } > > > > +/* > > + * To work around what seems to be an optimizer bug, the macro arguments > > + * need to have const copies or the values end up changed by the time they > > + * reach fortify_warn_once(). See commit 6f7630b1b5bc ("fortify: Capture > > + * __bos() results in const temp vars") for more details. > > + */ > > #define __fortify_memcpy_chk(p, q, size, p_size, q_size, \ > > p_size_field, q_size_field, op) ({ \ > > const size_t __fortify_size = (size_t)(size); \ > > @@ -623,6 +629,8 @@ __FORTIFY_INLINE bool fortify_memcpy_chk(__kernel_size_t size, > > const size_t __q_size = (q_size); \ > > const size_t __p_size_field = (p_size_field); \ > > const size_t __q_size_field = (q_size_field); \ > > + /* Keep a mutable version of the size for the final copy. */ \ > > + size_t __copy_size = __fortify_size; \ > > fortify_warn_once(fortify_memcpy_chk(__fortify_size, __p_size, \ > > __q_size, __p_size_field, \ > > __q_size_field, FORTIFY_FUNC_ ##op), \ > > @@ -630,7 +638,11 @@ __FORTIFY_INLINE bool fortify_memcpy_chk(__kernel_size_t size, > > __fortify_size, \ > > "field \"" #p "\" at " FILE_LINE, \ > > __p_size_field); \ > > - __underlying_##op(p, q, __fortify_size); \ > > + /* Hide only the run-time size from value range tracking to */ \ > > + /* silence compile-time false positive bounds warnings. */ \ > > + if (!__builtin_constant_p(__fortify_size)) \ > > + OPTIMIZER_HIDE_VAR(__copy_size); \ > > I think you can make that: > if (!__builtin_constant_p(__copy_size)) \ > OPTIMISER_HIDE_VAR(__copy_size) \ > which is probably more readable. Yeah, that tests out fine. I've updated it locally. Thanks! -Kees
diff --git a/include/linux/fortify-string.h b/include/linux/fortify-string.h index 0d99bf11d260..1eef0119671c 100644 --- a/include/linux/fortify-string.h +++ b/include/linux/fortify-string.h @@ -616,6 +616,12 @@ __FORTIFY_INLINE bool fortify_memcpy_chk(__kernel_size_t size, return false; } +/* + * To work around what seems to be an optimizer bug, the macro arguments + * need to have const copies or the values end up changed by the time they + * reach fortify_warn_once(). See commit 6f7630b1b5bc ("fortify: Capture + * __bos() results in const temp vars") for more details. + */ #define __fortify_memcpy_chk(p, q, size, p_size, q_size, \ p_size_field, q_size_field, op) ({ \ const size_t __fortify_size = (size_t)(size); \ @@ -623,6 +629,8 @@ __FORTIFY_INLINE bool fortify_memcpy_chk(__kernel_size_t size, const size_t __q_size = (q_size); \ const size_t __p_size_field = (p_size_field); \ const size_t __q_size_field = (q_size_field); \ + /* Keep a mutable version of the size for the final copy. */ \ + size_t __copy_size = __fortify_size; \ fortify_warn_once(fortify_memcpy_chk(__fortify_size, __p_size, \ __q_size, __p_size_field, \ __q_size_field, FORTIFY_FUNC_ ##op), \ @@ -630,7 +638,11 @@ __FORTIFY_INLINE bool fortify_memcpy_chk(__kernel_size_t size, __fortify_size, \ "field \"" #p "\" at " FILE_LINE, \ __p_size_field); \ - __underlying_##op(p, q, __fortify_size); \ + /* Hide only the run-time size from value range tracking to */ \ + /* silence compile-time false positive bounds warnings. */ \ + if (!__builtin_constant_p(__fortify_size)) \ + OPTIMIZER_HIDE_VAR(__copy_size); \ + __underlying_##op(p, q, __copy_size); \ }) /*
GCC performs value range tracking for variables as a way to provide better diagnostics. One place this is regularly seen is with warnings associated with bounds-checking, e.g. -Wstringop-overflow, -Wstringop-overread, -Warray-bounds, etc. In order to keep the signal-to-noise ratio high, warnings aren't emitted when a value range spans the entire value range representable by a given variable. For example: unsigned int len; char dst[8]; ... memcpy(dst, src, len); If len's value is unknown, it has the full "unsigned int" range of [0, UINT_MAX], and bounds checks against memcpy() will be ignored. However, when a code path has been able to narrow the range: if (len > 16) return; memcpy(dst, src, len); Then a range will be updated for the execution path. Above, len is now [0, 16], so we might see a -Wstringop-overflow warning like: error: '__builtin_memcpy' writing between 9 and 16 bytes from to region of size 8 [-Werror=stringop-overflow] When building with CONFIG_FORTIFY_SOURCE, the run-time bounds checking can appear to narrow value ranges for lengths for memcpy(), depending on how the compile constructs the execution paths during optimization passes, due to the checks on the size. For example: if (p_size_field != SIZE_MAX && p_size != p_size_field && p_size_field < size) As intentionally designed, these checks only affect the kernel warnings emitted at run-time and do not block the potentially overflowing memcpy(), so GCC thinks it needs to produce a warning about the resulting value range that might be reaching the memcpy(). We have seen this manifest a few times now, with the most recent being with cpumasks: In function ‘bitmap_copy’, inlined from ‘cpumask_copy’ at ./include/linux/cpumask.h:839:2, inlined from ‘__padata_set_cpumasks’ at kernel/padata.c:730:2: ./include/linux/fortify-string.h:114:33: error: ‘__builtin_memcpy’ reading between 257 and 536870904 bytes from a region of size 256 [-Werror=stringop-overread] 114 | #define __underlying_memcpy __builtin_memcpy | ^ ./include/linux/fortify-string.h:633:9: note: in expansion of macro ‘__underlying_memcpy’ 633 | __underlying_##op(p, q, __fortify_size); \ | ^~~~~~~~~~~~~ ./include/linux/fortify-string.h:678:26: note: in expansion of macro ‘__fortify_memcpy_chk’ 678 | #define memcpy(p, q, s) __fortify_memcpy_chk(p, q, s, \ | ^~~~~~~~~~~~~~~~~~~~ ./include/linux/bitmap.h:259:17: note: in expansion of macro ‘memcpy’ 259 | memcpy(dst, src, len); | ^~~~~~ kernel/padata.c: In function ‘__padata_set_cpumasks’: kernel/padata.c:713:48: note: source object ‘pcpumask’ of size [0, 256] 713 | cpumask_var_t pcpumask, | ~~~~~~~~~~~~~~^~~~~~~~ This warning is _not_ emitted when CONFIG_FORTIFY_SOURCE is disabled, and with the recent -fdiagnostics-details we can confirm the origin of the warning is due to the FORTIFY range checking: ../include/linux/bitmap.h:259:17: note: in expansion of macro 'memcpy' 259 | memcpy(dst, src, len); | ^~~~~~ '__padata_set_cpumasks': events 1-2 ../include/linux/fortify-string.h:613:36: 612 | if (p_size_field != SIZE_MAX && | ~~~~~~~~~~~~~~~~~~~~~~~~~~~ 613 | p_size != p_size_field && p_size_field < size) | ~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~ | | | (1) when the condition is evaluated to false | (2) when the condition is evaluated to true '__padata_set_cpumasks': event 3 114 | #define __underlying_memcpy __builtin_memcpy | ^ | | | (3) out of array bounds here Note that this warning started appearing since bitmap functions were recently marked __always_inline in commit ed8cd2b3bd9f ("bitmap: Switch from inline to __always_inline"), which allowed GCC to gain visibility into the variables as they passed through the FORTIFY implementation. In order to silence this false positive but keep deterministic compile-time warnings intact, hide the length variable from GCC with OPTIMIZE_HIDE_VAR() before calling the builtin memcpy. Additionally add a comment about why all the macro args have copies with const storage. Reported-by: "Thomas Weißschuh" <linux@weissschuh.net> Closes: https://lore.kernel.org/all/db7190c8-d17f-4a0d-bc2f-5903c79f36c2@t-8ch.de/ Reported-by: Nilay Shroff <nilay@linux.ibm.com> Closes: https://lore.kernel.org/all/20241112124127.1666300-1-nilay@linux.ibm.com/ Acked-by: Yury Norov <yury.norov@gmail.com> Signed-off-by: Kees Cook <kees@kernel.org> --- Cc: Nathan Chancellor <nathan@kernel.org> Cc: "Qing Zhao" <qing.zhao@oracle.com> Cc: linux-hardening@vger.kernel.org v2: Make sure the expression statement ends with a single statement v1: https://lore.kernel.org/all/20241213020929.work.498-kees@kernel.org/ --- include/linux/fortify-string.h | 14 +++++++++++++- 1 file changed, 13 insertions(+), 1 deletion(-)