[v2,1/5] uaccess: Add user_read_access_begin/end and user_write_access_begin/end

Commit Message

Christophe Leroy April 3, 2020, 7:20 a.m. UTC

Some architectures like powerpc64 have the capability to separate
read access and write access protection.
For get_user() and copy_from_user(), powerpc64 only open read access.
For put_user() and copy_to_user(), powerpc64 only open write access.
But when using unsafe_get_user() or unsafe_put_user(),
user_access_begin open both read and write.

Other architectures like powerpc book3s 32 bits only allow write
access protection. And on this architecture protection is an heavy
operation as it requires locking/unlocking per segment of 256Mbytes.
On those architecture it is therefore desirable to do the unlocking
only for write access. (Note that book3s/32 ranges from very old
powermac from the 90's with powerpc 601 processor, till modern
ADSL boxes with PowerQuicc II processors for instance so it
is still worth considering.)

In order to avoid any risk based of hacking some variable parameters
passed to user_access_begin/end that would allow hacking and
leaving user access open or opening too much, it is preferable to
use dedicated static functions that can't be overridden.

Add a user_read_access_begin and user_read_access_end to only open
read access.

Add a user_write_access_begin and user_write_access_end to only open
write access.

By default, when undefined, those new access helpers default on the
existing user_access_begin and user_access_end.

Signed-off-by: Christophe Leroy <[e-mail]>
Reviewed-by: Kees Cook <[e-mail]>
---
v2: no change in this patch. See each patch for related changes.

v1 at https://patchwork.ozlabs.org/project/linuxppc-dev/list/?series=168174

This series is based on the discussion we had in January, see
https://patchwork.ozlabs.org/patch/1227926/ . I tried to
take into account all remarks, especially @hpa 's remark to use
a fixed API on not base the relocking on a magic id returned at
unlocking.

This series is awaited for implementing selective lkdtm test to
test powerpc64 independant read and write protection, see
https://patchwork.ozlabs.org/patch/1231765/

Signed-off-by: Christophe Leroy <[e-mail]>
---
 include/linux/uaccess.h | 8 ++++++++
 1 file changed, 8 insertions(+)

Comments

Michael Ellerman May 29, 2020, 4:20 a.m. UTC | #1

On Fri, 2020-04-03 at 07:20:50 UTC, Christophe Leroy wrote:
> Some architectures like powerpc64 have the capability to separate
> read access and write access protection.
> For get_user() and copy_from_user(), powerpc64 only open read access.
> For put_user() and copy_to_user(), powerpc64 only open write access.
> But when using unsafe_get_user() or unsafe_put_user(),
> user_access_begin open both read and write.
> 
> Other architectures like powerpc book3s 32 bits only allow write
> access protection. And on this architecture protection is an heavy
> operation as it requires locking/unlocking per segment of 256Mbytes.
> On those architecture it is therefore desirable to do the unlocking
> only for write access. (Note that book3s/32 ranges from very old
> powermac from the 90's with powerpc 601 processor, till modern
> ADSL boxes with PowerQuicc II processors for instance so it
> is still worth considering.)
> 
> In order to avoid any risk based of hacking some variable parameters
> passed to user_access_begin/end that would allow hacking and
> leaving user access open or opening too much, it is preferable to
> use dedicated static functions that can't be overridden.
> 
> Add a user_read_access_begin and user_read_access_end to only open
> read access.
> 
> Add a user_write_access_begin and user_write_access_end to only open
> write access.
> 
> By default, when undefined, those new access helpers default on the
> existing user_access_begin and user_access_end.
> 
> Signed-off-by: Christophe Leroy <[e-mail]>
> Reviewed-by: Kees Cook <[e-mail]>

Applied to powerpc topic/uaccess, thanks.

https://git.kernel.org/powerpc/c/999a22890cb183b918e4372395d24426a755cef2

cheers

Patch

diff --git a/include/linux/uaccess.h b/include/linux/uaccess.h
index 67f016010aad..9861c89f93be 100644
--- a/include/linux/uaccess.h
+++ b/include/linux/uaccess.h
@@ -378,6 +378,14 @@  extern long strnlen_unsafe_user(const void __user *unsafe_addr, long count);
 static inline unsigned long user_access_save(void) { return 0UL; }
 static inline void user_access_restore(unsigned long flags) { }
 #endif
+#ifndef user_write_access_begin
+#define user_write_access_begin user_access_begin
+#define user_write_access_end user_access_end
+#endif
+#ifndef user_read_access_begin
+#define user_read_access_begin user_access_begin
+#define user_read_access_end user_access_end
+#endif
 
 #ifdef CONFIG_HARDENED_USERCOPY
 void usercopy_warn(const char *name, const char *detail, bool to_user,