[PATCH] x86/boot: Disable jump tables in PIC code

[PATCH] x86/boot: Disable jump tables in PIC code

[Ard Biesheuvel]

From: Ard Biesheuvel <ardb@kernel.org>

objtool already struggles to identify jump tables correctly in non-PIC
code, where the idiom is something like

  jmpq  *table(,%idx,8)

and the table is a list of absolute addresses of jump targets.

When using -fPIC, both the table reference as well as the jump targets
are emitted in a RIP-relative manner, resulting in something like

  leaq    table(%rip), %tbl
  movslq  (%tbl,%idx,4), %offset
  addq    %offset, %tbl
  jmpq    *%tbl

and the table is a list of offsets of the jump targets relative to the
start of the entire table.

Considering that this sequence of instructions can be interleaved with
other instructions that have nothing to do with the jump table in
question, it is extremely difficult to infer the control flow by
deriving the jump targets from the indirect jump, the location of the
table and the relative offsets it contains.

So let's not bother and disable jump tables for code built with -fPIC
under arch/x86/boot/startup.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
 arch/x86/boot/startup/Makefile | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/x86/boot/startup/Makefile b/arch/x86/boot/startup/Makefile
index e5f77e65c083..4062582144f6 100644
--- a/arch/x86/boot/startup/Makefile
+++ b/arch/x86/boot/startup/Makefile
@@ -5,6 +5,7 @@ KBUILD_CFLAGS		+= -D__DISABLE_EXPORTS -mcmodel=small -fPIC \
 			   -Os -DDISABLE_BRANCH_PROFILING \
 			   $(DISABLE_STACKLEAK_PLUGIN) \
 			   -fno-stack-protector -D__NO_FORTIFY \
+			   -fno-jump-tables \
 			   -include $(srctree)/include/linux/hidden.h
 
 # disable ftrace hooks and LTO

base-commit: ff4c0560ab020d34baf0aa6434f66333d25ae524
-- 
2.49.0.805.g082f7c87e0-goog

[tip: x86/boot] x86/boot: Disable jump tables in PIC code

[tip-bot2 for Ard Biesheuvel]

The following commit has been merged into the x86/boot branch of tip:

Commit-ID:     121c335b36e02d6aefb72501186e060474fdf33c
Gitweb:        https://git.kernel.org/tip/121c335b36e02d6aefb72501186e060474fdf33c
Author:        Ard Biesheuvel <ardb@kernel.org>
AuthorDate:    Tue, 22 Apr 2025 23:05:11 +02:00
Committer:     Ingo Molnar <mingo@kernel.org>
CommitterDate: Wed, 23 Apr 2025 09:30:57 +02:00

x86/boot: Disable jump tables in PIC code

objtool already struggles to identify jump tables correctly in non-PIC
code, where the idiom is something like

  jmpq  *table(,%idx,8)

and the table is a list of absolute addresses of jump targets.

When using -fPIC, both the table reference as well as the jump targets
are emitted in a RIP-relative manner, resulting in something like

  leaq    table(%rip), %tbl
  movslq  (%tbl,%idx,4), %offset
  addq    %offset, %tbl
  jmpq    *%tbl

and the table is a list of offsets of the jump targets relative to the
start of the entire table.

Considering that this sequence of instructions can be interleaved with
other instructions that have nothing to do with the jump table in
question, it is extremely difficult to infer the control flow by
deriving the jump targets from the indirect jump, the location of the
table and the relative offsets it contains.

So let's not bother and disable jump tables for code built with -fPIC
under arch/x86/boot/startup.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: linux-efi@vger.kernel.org
Link: https://lore.kernel.org/r/20250422210510.600354-2-ardb+git@google.com
---
 arch/x86/boot/startup/Makefile | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/x86/boot/startup/Makefile b/arch/x86/boot/startup/Makefile
index 21d911b..b514f7e 100644
--- a/arch/x86/boot/startup/Makefile
+++ b/arch/x86/boot/startup/Makefile
@@ -5,6 +5,7 @@ KBUILD_CFLAGS		+= -D__DISABLE_EXPORTS -mcmodel=small -fPIC \
 			   -Os -DDISABLE_BRANCH_PROFILING \
 			   $(DISABLE_STACKLEAK_PLUGIN) \
 			   -fno-stack-protector -D__NO_FORTIFY \
+			   -fno-jump-tables \
 			   -include $(srctree)/include/linux/hidden.h
 
 # disable ftrace hooks and LTO

Re: [tip: x86/boot] x86/boot: Disable jump tables in PIC code

[Josh Poimboeuf]

On Wed, Apr 23, 2025 at 07:48:01AM +0000, tip-bot2 for Ard Biesheuvel wrote:
> The following commit has been merged into the x86/boot branch of tip:
> 
> Commit-ID:     121c335b36e02d6aefb72501186e060474fdf33c
> Gitweb:        https://git.kernel.org/tip/121c335b36e02d6aefb72501186e060474fdf33c
> Author:        Ard Biesheuvel <ardb@kernel.org>
> AuthorDate:    Tue, 22 Apr 2025 23:05:11 +02:00
> Committer:     Ingo Molnar <mingo@kernel.org>
> CommitterDate: Wed, 23 Apr 2025 09:30:57 +02:00
> 
> x86/boot: Disable jump tables in PIC code
> 
> objtool already struggles to identify jump tables correctly in non-PIC
> code, where the idiom is something like
> 
>   jmpq  *table(,%idx,8)
> 
> and the table is a list of absolute addresses of jump targets.
> 
> When using -fPIC, both the table reference as well as the jump targets
> are emitted in a RIP-relative manner, resulting in something like
> 
>   leaq    table(%rip), %tbl
>   movslq  (%tbl,%idx,4), %offset
>   addq    %offset, %tbl
>   jmpq    *%tbl
> 
> and the table is a list of offsets of the jump targets relative to the
> start of the entire table.
> 
> Considering that this sequence of instructions can be interleaved with
> other instructions that have nothing to do with the jump table in
> question, it is extremely difficult to infer the control flow by
> deriving the jump targets from the indirect jump, the location of the
> table and the relative offsets it contains.
> 
> So let's not bother and disable jump tables for code built with -fPIC
> under arch/x86/boot/startup.

Hm, does objtool even run on boot code?

-- 
Josh

Re: [tip: x86/boot] x86/boot: Disable jump tables in PIC code

[Ard Biesheuvel]

On Wed, 23 Apr 2025 at 16:55, Josh Poimboeuf <jpoimboe@kernel.org> wrote:
>
> On Wed, Apr 23, 2025 at 07:48:01AM +0000, tip-bot2 for Ard Biesheuvel wrote:
> > The following commit has been merged into the x86/boot branch of tip:
> >
> > Commit-ID:     121c335b36e02d6aefb72501186e060474fdf33c
> > Gitweb:        https://git.kernel.org/tip/121c335b36e02d6aefb72501186e060474fdf33c
> > Author:        Ard Biesheuvel <ardb@kernel.org>
> > AuthorDate:    Tue, 22 Apr 2025 23:05:11 +02:00
> > Committer:     Ingo Molnar <mingo@kernel.org>
> > CommitterDate: Wed, 23 Apr 2025 09:30:57 +02:00
> >
> > x86/boot: Disable jump tables in PIC code
> >
> > objtool already struggles to identify jump tables correctly in non-PIC
> > code, where the idiom is something like
> >
> >   jmpq  *table(,%idx,8)
> >
> > and the table is a list of absolute addresses of jump targets.
> >
> > When using -fPIC, both the table reference as well as the jump targets
> > are emitted in a RIP-relative manner, resulting in something like
> >
> >   leaq    table(%rip), %tbl
> >   movslq  (%tbl,%idx,4), %offset
> >   addq    %offset, %tbl
> >   jmpq    *%tbl
> >
> > and the table is a list of offsets of the jump targets relative to the
> > start of the entire table.
> >
> > Considering that this sequence of instructions can be interleaved with
> > other instructions that have nothing to do with the jump table in
> > question, it is extremely difficult to infer the control flow by
> > deriving the jump targets from the indirect jump, the location of the
> > table and the relative offsets it contains.
> >
> > So let's not bother and disable jump tables for code built with -fPIC
> > under arch/x86/boot/startup.
>
> Hm, does objtool even run on boot code?
>

This is about startup code, not boot code. This is code that is part
of vmlinux, but runs from a different mapping of memory than the one
the linker assumes, and so it needs to be built with -fPIC to
discourage the compiler and linker from inserting symbol references
via the kernel virtual mapping, which may not be up yet when this code
runs.

Re: [tip: x86/boot] x86/boot: Disable jump tables in PIC code

[Josh Poimboeuf]

On Wed, Apr 23, 2025 at 05:01:42PM +0200, Ard Biesheuvel wrote:
> > > So let's not bother and disable jump tables for code built with -fPIC
> > > under arch/x86/boot/startup.
> >
> > Hm, does objtool even run on boot code?
> >
> 
> This is about startup code, not boot code. This is code that is part
> of vmlinux, but runs from a different mapping of memory than the one
> the linker assumes, and so it needs to be built with -fPIC to
> discourage the compiler and linker from inserting symbol references
> via the kernel virtual mapping, which may not be up yet when this code
> runs.

Maybe objtool should ignore .head.text.  It doesn't need ORC, static
calls, uaccess validation, retpolines, etc.

-- 
Josh

Re: [tip: x86/boot] x86/boot: Disable jump tables in PIC code

[Ard Biesheuvel]

On Wed, 23 Apr 2025 at 18:41, Josh Poimboeuf <jpoimboe@kernel.org> wrote:
>
> On Wed, Apr 23, 2025 at 05:01:42PM +0200, Ard Biesheuvel wrote:
> > > > So let's not bother and disable jump tables for code built with -fPIC
> > > > under arch/x86/boot/startup.
> > >
> > > Hm, does objtool even run on boot code?
> > >
> >
> > This is about startup code, not boot code. This is code that is part
> > of vmlinux, but runs from a different mapping of memory than the one
> > the linker assumes, and so it needs to be built with -fPIC to
> > discourage the compiler and linker from inserting symbol references
> > via the kernel virtual mapping, which may not be up yet when this code
> > runs.
>
> Maybe objtool should ignore .head.text.  It doesn't need ORC, static
> calls, uaccess validation, retpolines, etc.
>

I am trying to get rid of .head.text.

But some of the startup code may still be in use later, so I don't
think we should disable objtool validation entirely unless we really
have to.