[PATCH] stack: replace "o" output with "r" input constraint

[PATCH] stack: replace "o" output with "r" input constraint

[Kees Cook]

From: Nick Desaulniers <ndesaulniers@google.com>

"o" isn't a common asm() constraint to use; it triggers an assertion in
assert-enabled builds of LLVM that it's not recognized when targeting
aarch64 (though it appears to fall back to "m"). I've fixed this in LLVM
13 now, but there isn't really a good reason to be using "o" in particular
here. To avoid causing build issues for those using assert-enabled builds
of earlier LLVM versions, the constraint needs changing.

Instead, if the point is to retain the __builtin_alloca(), we can make ptr
appear to "escape" via being an input to an empty inline asm block. This
is preferable anyways, since otherwise this looks like a dead store.

While the use of "r" was considered in
https://lore.kernel.org/lkml/202104011447.2E7F543@keescook/
it was only tested as an output (which looks like a dead store, and
wasn't sufficient). Use "r" as an input constraint instead, which
behaves correctly across compilers and architectures:
https://godbolt.org/z/E9cd411ob

Link: https://reviews.llvm.org/D100412
Link: https://bugs.llvm.org/show_bug.cgi?id=49956
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Tested-by: Kees Cook <keescook@chromium.org>
Fixes: 39218ff4c625 ("stack: Optionally randomize kernel stack offset each syscall")
Signed-off-by: Kees Cook <keescook@chromium.org>
---
 include/linux/randomize_kstack.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/randomize_kstack.h b/include/linux/randomize_kstack.h
index fd80fab663a9..bebc911161b6 100644
--- a/include/linux/randomize_kstack.h
+++ b/include/linux/randomize_kstack.h
@@ -38,7 +38,7 @@ void *__builtin_alloca(size_t size);
 		u32 offset = raw_cpu_read(kstack_offset);		\
 		u8 *ptr = __builtin_alloca(KSTACK_OFFSET_MAX(offset));	\
 		/* Keep allocation even after "ptr" loses scope. */	\
-		asm volatile("" : "=o"(*ptr) :: "memory");		\
+		asm volatile("" :: "r"(ptr) : "memory");		\
 	}								\
 } while (0)
 
-- 
2.25.1

Re: [PATCH] stack: replace "o" output with "r" input constraint

[Nathan Chancellor]

On Mon, Apr 19, 2021 at 04:17:41PM -0700, Kees Cook wrote:
> From: Nick Desaulniers <ndesaulniers@google.com>
> 
> "o" isn't a common asm() constraint to use; it triggers an assertion in
> assert-enabled builds of LLVM that it's not recognized when targeting
> aarch64 (though it appears to fall back to "m"). I've fixed this in LLVM
> 13 now, but there isn't really a good reason to be using "o" in particular
> here. To avoid causing build issues for those using assert-enabled builds
> of earlier LLVM versions, the constraint needs changing.
> 
> Instead, if the point is to retain the __builtin_alloca(), we can make ptr
> appear to "escape" via being an input to an empty inline asm block. This
> is preferable anyways, since otherwise this looks like a dead store.
> 
> While the use of "r" was considered in
> https://lore.kernel.org/lkml/202104011447.2E7F543@keescook/
> it was only tested as an output (which looks like a dead store, and
> wasn't sufficient). Use "r" as an input constraint instead, which
> behaves correctly across compilers and architectures:
> https://godbolt.org/z/E9cd411ob
> 
> Link: https://reviews.llvm.org/D100412
> Link: https://bugs.llvm.org/show_bug.cgi?id=49956
> Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
> Tested-by: Kees Cook <keescook@chromium.org>
> Fixes: 39218ff4c625 ("stack: Optionally randomize kernel stack offset each syscall")
> Signed-off-by: Kees Cook <keescook@chromium.org>

I built arm64 defconfig with and without
CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT with LLVM 12 (which does not have
Nick's LLVM fix) without any issues and did a quick boot test in QEMU,
nothing exploded.

Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Tested-by: Nathan Chancellor <nathan@kernel.org>

> ---
>  include/linux/randomize_kstack.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/randomize_kstack.h b/include/linux/randomize_kstack.h
> index fd80fab663a9..bebc911161b6 100644
> --- a/include/linux/randomize_kstack.h
> +++ b/include/linux/randomize_kstack.h
> @@ -38,7 +38,7 @@ void *__builtin_alloca(size_t size);
>  		u32 offset = raw_cpu_read(kstack_offset);		\
>  		u8 *ptr = __builtin_alloca(KSTACK_OFFSET_MAX(offset));	\
>  		/* Keep allocation even after "ptr" loses scope. */	\
> -		asm volatile("" : "=o"(*ptr) :: "memory");		\
> +		asm volatile("" :: "r"(ptr) : "memory");		\
>  	}								\
>  } while (0)
>  
> -- 
> 2.25.1
> 

Re: [PATCH] stack: replace "o" output with "r" input constraint

[Nathan Chancellor]

On Thu, Apr 22, 2021 at 06:02:27PM -0700, Nathan Chancellor wrote:
> On Mon, Apr 19, 2021 at 04:17:41PM -0700, Kees Cook wrote:
> > From: Nick Desaulniers <ndesaulniers@google.com>
> > 
> > "o" isn't a common asm() constraint to use; it triggers an assertion in
> > assert-enabled builds of LLVM that it's not recognized when targeting
> > aarch64 (though it appears to fall back to "m"). I've fixed this in LLVM
> > 13 now, but there isn't really a good reason to be using "o" in particular
> > here. To avoid causing build issues for those using assert-enabled builds
> > of earlier LLVM versions, the constraint needs changing.
> > 
> > Instead, if the point is to retain the __builtin_alloca(), we can make ptr
> > appear to "escape" via being an input to an empty inline asm block. This
> > is preferable anyways, since otherwise this looks like a dead store.
> > 
> > While the use of "r" was considered in
> > https://lore.kernel.org/lkml/202104011447.2E7F543@keescook/
> > it was only tested as an output (which looks like a dead store, and
> > wasn't sufficient). Use "r" as an input constraint instead, which
> > behaves correctly across compilers and architectures:
> > https://godbolt.org/z/E9cd411ob
> > 
> > Link: https://reviews.llvm.org/D100412
> > Link: https://bugs.llvm.org/show_bug.cgi?id=49956
> > Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
> > Tested-by: Kees Cook <keescook@chromium.org>
> > Fixes: 39218ff4c625 ("stack: Optionally randomize kernel stack offset each syscall")

Kees, were you planning on taking this to Linus or someone else? It
would be nice to have this in for -rc1 (although I understand it might
be too late), if not, by -rc2.

Cheers,
Nathan

> > Signed-off-by: Kees Cook <keescook@chromium.org>
> 
> I built arm64 defconfig with and without
> CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT with LLVM 12 (which does not have
> Nick's LLVM fix) without any issues and did a quick boot test in QEMU,
> nothing exploded.
> 
> Reviewed-by: Nathan Chancellor <nathan@kernel.org>
> Tested-by: Nathan Chancellor <nathan@kernel.org>
> 
> > ---
> >  include/linux/randomize_kstack.h | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/include/linux/randomize_kstack.h b/include/linux/randomize_kstack.h
> > index fd80fab663a9..bebc911161b6 100644
> > --- a/include/linux/randomize_kstack.h
> > +++ b/include/linux/randomize_kstack.h
> > @@ -38,7 +38,7 @@ void *__builtin_alloca(size_t size);
> >  		u32 offset = raw_cpu_read(kstack_offset);		\
> >  		u8 *ptr = __builtin_alloca(KSTACK_OFFSET_MAX(offset));	\
> >  		/* Keep allocation even after "ptr" loses scope. */	\
> > -		asm volatile("" : "=o"(*ptr) :: "memory");		\
> > +		asm volatile("" :: "r"(ptr) : "memory");		\
> >  	}								\
> >  } while (0)
> >  
> > -- 
> > 2.25.1
> > 

Re: [PATCH] stack: replace "o" output with "r" input constraint

[Kees Cook]

On Wed, May 05, 2021 at 02:12:32PM -0700, Nathan Chancellor wrote:
> On Thu, Apr 22, 2021 at 06:02:27PM -0700, Nathan Chancellor wrote:
> > On Mon, Apr 19, 2021 at 04:17:41PM -0700, Kees Cook wrote:
> > > From: Nick Desaulniers <ndesaulniers@google.com>
> > > 
> > > "o" isn't a common asm() constraint to use; it triggers an assertion in
> > > assert-enabled builds of LLVM that it's not recognized when targeting
> > > aarch64 (though it appears to fall back to "m"). I've fixed this in LLVM
> > > 13 now, but there isn't really a good reason to be using "o" in particular
> > > here. To avoid causing build issues for those using assert-enabled builds
> > > of earlier LLVM versions, the constraint needs changing.
> > > 
> > > Instead, if the point is to retain the __builtin_alloca(), we can make ptr
> > > appear to "escape" via being an input to an empty inline asm block. This
> > > is preferable anyways, since otherwise this looks like a dead store.
> > > 
> > > While the use of "r" was considered in
> > > https://lore.kernel.org/lkml/202104011447.2E7F543@keescook/
> > > it was only tested as an output (which looks like a dead store, and
> > > wasn't sufficient). Use "r" as an input constraint instead, which
> > > behaves correctly across compilers and architectures:
> > > https://godbolt.org/z/E9cd411ob
> > > 
> > > Link: https://reviews.llvm.org/D100412
> > > Link: https://bugs.llvm.org/show_bug.cgi?id=49956
> > > Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
> > > Tested-by: Kees Cook <keescook@chromium.org>
> > > Fixes: 39218ff4c625 ("stack: Optionally randomize kernel stack offset each syscall")
> 
> Kees, were you planning on taking this to Linus or someone else? It
> would be nice to have this in for -rc1 (although I understand it might
> be too late), if not, by -rc2.

I assumed Thomas would pick this up. Thomas, shall I send this directly
to Linus?

Thanks!

-Kees

> 
> Cheers,
> Nathan
> 
> > > Signed-off-by: Kees Cook <keescook@chromium.org>
> > 
> > I built arm64 defconfig with and without
> > CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT with LLVM 12 (which does not have
> > Nick's LLVM fix) without any issues and did a quick boot test in QEMU,
> > nothing exploded.
> > 
> > Reviewed-by: Nathan Chancellor <nathan@kernel.org>
> > Tested-by: Nathan Chancellor <nathan@kernel.org>
> > 
> > > ---
> > >  include/linux/randomize_kstack.h | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/include/linux/randomize_kstack.h b/include/linux/randomize_kstack.h
> > > index fd80fab663a9..bebc911161b6 100644
> > > --- a/include/linux/randomize_kstack.h
> > > +++ b/include/linux/randomize_kstack.h
> > > @@ -38,7 +38,7 @@ void *__builtin_alloca(size_t size);
> > >  		u32 offset = raw_cpu_read(kstack_offset);		\
> > >  		u8 *ptr = __builtin_alloca(KSTACK_OFFSET_MAX(offset));	\
> > >  		/* Keep allocation even after "ptr" loses scope. */	\
> > > -		asm volatile("" : "=o"(*ptr) :: "memory");		\
> > > +		asm volatile("" :: "r"(ptr) : "memory");		\
> > >  	}								\
> > >  } while (0)
> > >  
> > > -- 
> > > 2.25.1
> > > 

-- 
Kees Cook

[tip: core/urgent] stack: Replace "o" output with "r" input constraint

[tip-bot2 for Nick Desaulniers]

The following commit has been merged into the core/urgent branch of tip:

Commit-ID:     2515dd6ce8e545b0b2eece84920048ef9ed846c4
Gitweb:        https://git.kernel.org/tip/2515dd6ce8e545b0b2eece84920048ef9ed846c4
Author:        Nick Desaulniers <ndesaulniers@google.com>
AuthorDate:    Mon, 19 Apr 2021 16:17:41 -07:00
Committer:     Thomas Gleixner <tglx@linutronix.de>
CommitterDate: Tue, 11 May 2021 09:56:11 +02:00

stack: Replace "o" output with "r" input constraint

"o" isn't a common asm() constraint to use; it triggers an assertion in
assert-enabled builds of LLVM that it's not recognized when targeting
aarch64 (though it appears to fall back to "m"). It's fixed in LLVM 13 now,
but there isn't really a good reason to use "o" in particular here. To
avoid causing build issues for those using assert-enabled builds of earlier
LLVM versions, the constraint needs changing.

Instead, if the point is to retain the __builtin_alloca(), make ptr appear
to "escape" via being an input to an empty inline asm block. This is
preferable anyways, since otherwise this looks like a dead store.

While the use of "r" was considered in

  https://lore.kernel.org/lkml/202104011447.2E7F543@keescook/

it was only tested as an output (which looks like a dead store, and wasn't
sufficient).

Use "r" as an input constraint instead, which behaves correctly across
compilers and architectures.

Fixes: 39218ff4c625 ("stack: Optionally randomize kernel stack offset each syscall")
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Kees Cook <keescook@chromium.org>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Link: https://reviews.llvm.org/D100412
Link: https://bugs.llvm.org/show_bug.cgi?id=49956
Link: https://lore.kernel.org/r/20210419231741.4084415-1-keescook@chromium.org

---
 include/linux/randomize_kstack.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/randomize_kstack.h b/include/linux/randomize_kstack.h
index fd80fab..bebc911 100644
--- a/include/linux/randomize_kstack.h
+++ b/include/linux/randomize_kstack.h
@@ -38,7 +38,7 @@ void *__builtin_alloca(size_t size);
 		u32 offset = raw_cpu_read(kstack_offset);		\
 		u8 *ptr = __builtin_alloca(KSTACK_OFFSET_MAX(offset));	\
 		/* Keep allocation even after "ptr" loses scope. */	\
-		asm volatile("" : "=o"(*ptr) :: "memory");		\
+		asm volatile("" :: "r"(ptr) : "memory");		\
 	}								\
 } while (0)