diff mbox series

[v2,4/5] x86/xen: Avoid relocatable quantities in Xen ELF notes

Message ID 20240930071513.909462-11-ardb+git@google.com (mailing list archive)
State Superseded
Headers show
Series x86/xen: Drop absolute references from startup code | expand

Commit Message

Ard Biesheuvel Sept. 30, 2024, 7:15 a.m. UTC 
From: Ard Biesheuvel <ardb@kernel.org>

Xen puts virtual and physical addresses into ELF notes that are treated
by the linker as relocatable by default. Doing so is not only pointless,
given that the ELF notes are only intended for consumption by Xen before
the kernel boots. It is also a KASLR leak, given that the kernel's ELF
notes are exposed via the world readable /sys/kernel/notes.

So emit these constants in a way that prevents the linker from marking
them as relocatable. This involves place-relative relocations (which
subtract their own virtual address from the symbol value) and linker
provided absolute symbols that add the address of the place to the
desired value.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
 arch/x86/kernel/vmlinux.lds.S | 13 +++++++++++++
 arch/x86/platform/pvh/head.S  |  6 +++---
 arch/x86/tools/relocs.c       |  1 +
 arch/x86/xen/xen-head.S       |  6 ++++--
 4 files changed, 21 insertions(+), 5 deletions(-)

Comments

Jason Andryuk Oct. 2, 2024, 9:25 p.m. UTC | #1 
On 2024-09-30 03:15, Ard Biesheuvel wrote:
> From: Ard Biesheuvel <ardb@kernel.org>
> 
> Xen puts virtual and physical addresses into ELF notes that are treated
> by the linker as relocatable by default. Doing so is not only pointless,
> given that the ELF notes are only intended for consumption by Xen before
> the kernel boots. It is also a KASLR leak, given that the kernel's ELF
> notes are exposed via the world readable /sys/kernel/notes.
> 
> So emit these constants in a way that prevents the linker from marking
> them as relocatable. This involves place-relative relocations (which
> subtract their own virtual address from the symbol value) and linker
> provided absolute symbols that add the address of the place to the
> desired value.
> 
> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>

Tested-by: Jason Andryuk <jason.andryuk@amd.com>

The generated values look ok.

> ---
>   arch/x86/kernel/vmlinux.lds.S | 13 +++++++++++++
>   arch/x86/platform/pvh/head.S  |  6 +++---
>   arch/x86/tools/relocs.c       |  1 +
>   arch/x86/xen/xen-head.S       |  6 ++++--
>   4 files changed, 21 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S
> index 6726be89b7a6..2b7c8c14c6fd 100644
> --- a/arch/x86/kernel/vmlinux.lds.S
> +++ b/arch/x86/kernel/vmlinux.lds.S
> @@ -527,3 +527,16 @@ INIT_PER_CPU(irq_stack_backing_store);
>   #endif
>   
>   #endif /* CONFIG_X86_64 */
> +
> +#ifdef CONFIG_XEN
> +#ifdef CONFIG_XEN_PV
> +xen_elfnote_entry_offset =
> +	ABSOLUTE(xen_elfnote_entry) + ABSOLUTE(startup_xen);
> +#endif
> +xen_elfnote_hypercall_page_offset =
> +	ABSOLUTE(xen_elfnote_hypercall_page) + ABSOLUTE(hypercall_page);
> +#endif
> +#ifdef CONFIG_PVH
> +xen_elfnote_phys32_entry_offset =
> +	ABSOLUTE(xen_elfnote_phys32_entry) + ABSOLUTE(pvh_start_xen - LOAD_OFFSET);
> +#endif

It seems to me, these aren't really offsets, but instead an address + value.

> diff --git a/arch/x86/platform/pvh/head.S b/arch/x86/platform/pvh/head.S
> index 7ca51a4da217..2b0d887e0872 100644
> --- a/arch/x86/platform/pvh/head.S
> +++ b/arch/x86/platform/pvh/head.S

> @@ -300,5 +300,5 @@ SYM_DATA_END(pvh_level2_kernel_pgt)
>   		     .long KERNEL_IMAGE_SIZE - 1)
>   #endif
>   
> -	ELFNOTE(Xen, XEN_ELFNOTE_PHYS32_ENTRY,
> -	             _ASM_PTR (pvh_start_xen - __START_KERNEL_map))
> +	ELFNOTE(Xen, XEN_ELFNOTE_PHYS32_ENTRY, .global xen_elfnote_phys32_entry;
> +		xen_elfnote_phys32_entry: _ASM_PTR xen_elfnote_phys32_entry_offset - .)

So here you have `address + value - address` to put the desired value in 
the elf note?

Regards,
Jason
Ard Biesheuvel Oct. 3, 2024, 7:49 a.m. UTC | #2 
On Wed, 2 Oct 2024 at 23:25, Jason Andryuk <jason.andryuk@amd.com> wrote:
>
> On 2024-09-30 03:15, Ard Biesheuvel wrote:
> > From: Ard Biesheuvel <ardb@kernel.org>
> >
> > Xen puts virtual and physical addresses into ELF notes that are treated
> > by the linker as relocatable by default. Doing so is not only pointless,
> > given that the ELF notes are only intended for consumption by Xen before
> > the kernel boots. It is also a KASLR leak, given that the kernel's ELF
> > notes are exposed via the world readable /sys/kernel/notes.
> >
> > So emit these constants in a way that prevents the linker from marking
> > them as relocatable. This involves place-relative relocations (which
> > subtract their own virtual address from the symbol value) and linker
> > provided absolute symbols that add the address of the place to the
> > desired value.
> >
> > Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
>
> Tested-by: Jason Andryuk <jason.andryuk@amd.com>
>
> The generated values look ok.
>
> > ---
> >   arch/x86/kernel/vmlinux.lds.S | 13 +++++++++++++
> >   arch/x86/platform/pvh/head.S  |  6 +++---
> >   arch/x86/tools/relocs.c       |  1 +
> >   arch/x86/xen/xen-head.S       |  6 ++++--
> >   4 files changed, 21 insertions(+), 5 deletions(-)
> >
> > diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S
> > index 6726be89b7a6..2b7c8c14c6fd 100644
> > --- a/arch/x86/kernel/vmlinux.lds.S
> > +++ b/arch/x86/kernel/vmlinux.lds.S
> > @@ -527,3 +527,16 @@ INIT_PER_CPU(irq_stack_backing_store);
> >   #endif
> >
> >   #endif /* CONFIG_X86_64 */
> > +
> > +#ifdef CONFIG_XEN
> > +#ifdef CONFIG_XEN_PV
> > +xen_elfnote_entry_offset =
> > +     ABSOLUTE(xen_elfnote_entry) + ABSOLUTE(startup_xen);
> > +#endif
> > +xen_elfnote_hypercall_page_offset =
> > +     ABSOLUTE(xen_elfnote_hypercall_page) + ABSOLUTE(hypercall_page);
> > +#endif
> > +#ifdef CONFIG_PVH
> > +xen_elfnote_phys32_entry_offset =
> > +     ABSOLUTE(xen_elfnote_phys32_entry) + ABSOLUTE(pvh_start_xen - LOAD_OFFSET);
> > +#endif
>
> It seems to me, these aren't really offsets, but instead an address + value.
>

Indeed. So xen_elfnote_phys32_entry_value would probably be a better name.

> > diff --git a/arch/x86/platform/pvh/head.S b/arch/x86/platform/pvh/head.S
> > index 7ca51a4da217..2b0d887e0872 100644
> > --- a/arch/x86/platform/pvh/head.S
> > +++ b/arch/x86/platform/pvh/head.S
>
> > @@ -300,5 +300,5 @@ SYM_DATA_END(pvh_level2_kernel_pgt)
> >                    .long KERNEL_IMAGE_SIZE - 1)
> >   #endif
> >
> > -     ELFNOTE(Xen, XEN_ELFNOTE_PHYS32_ENTRY,
> > -                  _ASM_PTR (pvh_start_xen - __START_KERNEL_map))
> > +     ELFNOTE(Xen, XEN_ELFNOTE_PHYS32_ENTRY, .global xen_elfnote_phys32_entry;
> > +             xen_elfnote_phys32_entry: _ASM_PTR xen_elfnote_phys32_entry_offset - .)
>
> So here you have `address + value - address` to put the desired value in
> the elf note?
>

Exactly. The assembler emits a relative relocation, and the linker
resolves it at build time.
diff mbox series

Patch

diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S
index 6726be89b7a6..2b7c8c14c6fd 100644
--- a/arch/x86/kernel/vmlinux.lds.S
+++ b/arch/x86/kernel/vmlinux.lds.S
@@ -527,3 +527,16 @@  INIT_PER_CPU(irq_stack_backing_store);
 #endif
 
 #endif /* CONFIG_X86_64 */
+
+#ifdef CONFIG_XEN
+#ifdef CONFIG_XEN_PV
+xen_elfnote_entry_offset =
+	ABSOLUTE(xen_elfnote_entry) + ABSOLUTE(startup_xen);
+#endif
+xen_elfnote_hypercall_page_offset =
+	ABSOLUTE(xen_elfnote_hypercall_page) + ABSOLUTE(hypercall_page);
+#endif
+#ifdef CONFIG_PVH
+xen_elfnote_phys32_entry_offset =
+	ABSOLUTE(xen_elfnote_phys32_entry) + ABSOLUTE(pvh_start_xen - LOAD_OFFSET);
+#endif
diff --git a/arch/x86/platform/pvh/head.S b/arch/x86/platform/pvh/head.S
index 7ca51a4da217..2b0d887e0872 100644
--- a/arch/x86/platform/pvh/head.S
+++ b/arch/x86/platform/pvh/head.S
@@ -52,7 +52,7 @@ 
 #define PVH_CS_SEL		(PVH_GDT_ENTRY_CS * 8)
 #define PVH_DS_SEL		(PVH_GDT_ENTRY_DS * 8)
 
-SYM_CODE_START_LOCAL(pvh_start_xen)
+SYM_CODE_START(pvh_start_xen)
 	UNWIND_HINT_END_OF_STACK
 	cld
 
@@ -300,5 +300,5 @@  SYM_DATA_END(pvh_level2_kernel_pgt)
 		     .long KERNEL_IMAGE_SIZE - 1)
 #endif
 
-	ELFNOTE(Xen, XEN_ELFNOTE_PHYS32_ENTRY,
-	             _ASM_PTR (pvh_start_xen - __START_KERNEL_map))
+	ELFNOTE(Xen, XEN_ELFNOTE_PHYS32_ENTRY, .global xen_elfnote_phys32_entry;
+		xen_elfnote_phys32_entry: _ASM_PTR xen_elfnote_phys32_entry_offset - .)
diff --git a/arch/x86/tools/relocs.c b/arch/x86/tools/relocs.c
index c101bed61940..3ede19ca8432 100644
--- a/arch/x86/tools/relocs.c
+++ b/arch/x86/tools/relocs.c
@@ -56,6 +56,7 @@  static const char * const	sym_regex_kernel[S_NSYMTYPES] = {
 	[S_ABS] =
 	"^(xen_irq_disable_direct_reloc$|"
 	"xen_save_fl_direct_reloc$|"
+	"xen_elfnote_.+_offset$|"
 	"VDSO|"
 	"__kcfi_typeid_|"
 	"__crc_)",
diff --git a/arch/x86/xen/xen-head.S b/arch/x86/xen/xen-head.S
index 758bcd47b72d..3deaae3601f7 100644
--- a/arch/x86/xen/xen-head.S
+++ b/arch/x86/xen/xen-head.S
@@ -94,7 +94,8 @@  SYM_CODE_END(xen_cpu_bringup_again)
 	ELFNOTE(Xen, XEN_ELFNOTE_VIRT_BASE,      _ASM_PTR __START_KERNEL_map)
 	/* Map the p2m table to a 512GB-aligned user address. */
 	ELFNOTE(Xen, XEN_ELFNOTE_INIT_P2M,       .quad (PUD_SIZE * PTRS_PER_PUD))
-	ELFNOTE(Xen, XEN_ELFNOTE_ENTRY,          _ASM_PTR startup_xen)
+	ELFNOTE(Xen, XEN_ELFNOTE_ENTRY,          .globl xen_elfnote_entry;
+		xen_elfnote_entry: _ASM_PTR xen_elfnote_entry_offset - .)
 	ELFNOTE(Xen, XEN_ELFNOTE_FEATURES,       .ascii "!writable_page_tables")
 	ELFNOTE(Xen, XEN_ELFNOTE_PAE_MODE,       .asciz "yes")
 	ELFNOTE(Xen, XEN_ELFNOTE_L1_MFN_VALID,
@@ -115,7 +116,8 @@  SYM_CODE_END(xen_cpu_bringup_again)
 #else
 # define FEATURES_DOM0 0
 #endif
-	ELFNOTE(Xen, XEN_ELFNOTE_HYPERCALL_PAGE, _ASM_PTR hypercall_page)
+	ELFNOTE(Xen, XEN_ELFNOTE_HYPERCALL_PAGE, .globl xen_elfnote_hypercall_page;
+		xen_elfnote_hypercall_page: _ASM_PTR xen_elfnote_hypercall_page_offset - .)
 	ELFNOTE(Xen, XEN_ELFNOTE_SUPPORTED_FEATURES,
 		.long FEATURES_PV | FEATURES_PVH | FEATURES_DOM0)
 	ELFNOTE(Xen, XEN_ELFNOTE_LOADER,         .asciz "generic")