[PATCH v2 0/2] ARM: ptrace: Restore syscall skipping and restart while tracing

[PATCH v2 0/2] ARM: ptrace: Restore syscall skipping and restart while tracing

[Kees Cook]

Hi,

Fix tracing on arm since commit 4e57a4ddf6b0 ("ARM: 9107/1: syscall:
always store thread_info->abi_syscall"). This was seen with the broken
seccomp tests "syscall_errno", "syscall_faked", and "syscall_restart".

Thanks!

-Kees

v2:
- split fixes
- move scno store into the actual restart path
v1: https://lore.kernel.org/lkml/20230804071045.never.134-kees@kernel.org

Kees Cook (2):
  ARM: ptrace: Restore syscall restart tracing
  ARM: ptrace: Restore syscall skipping for tracers

 arch/arm/include/asm/syscall.h | 3 +++
 arch/arm/kernel/entry-common.S | 1 +
 arch/arm/kernel/ptrace.c       | 5 +++--
 3 files changed, 7 insertions(+), 2 deletions(-)

-- 
2.34.1


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[PATCH v2 1/2] ARM: ptrace: Restore syscall restart tracing

[Kees Cook]

Since commit 4e57a4ddf6b0 ("ARM: 9107/1: syscall: always store
thread_info->abi_syscall"), the seccomp selftests "syscall_restart" has
been broken. This was caused by the restart syscall not being stored to
"abi_syscall" during restart setup before branching to the "local_restart"
label. Tracers would see the wrong syscall, and scno would get overwritten
while returning from the TIF_WORK path. Add the missing store.

Cc: Russell King <linux@armlinux.org.uk>
Cc: Arnd Bergmann <arnd@kernel.org>
Cc: Lecopzer Chen <lecopzer.chen@mediatek.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: linux-arm-kernel@lists.infradead.org
Fixes: 4e57a4ddf6b0 ("ARM: 9107/1: syscall: always store thread_info->abi_syscall")
Signed-off-by: Kees Cook <keescook@chromium.org>
---
 arch/arm/kernel/entry-common.S | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/arm/kernel/entry-common.S b/arch/arm/kernel/entry-common.S
index bcc4c9ec3aa4..5c31e9de7a60 100644
--- a/arch/arm/kernel/entry-common.S
+++ b/arch/arm/kernel/entry-common.S
@@ -90,6 +90,7 @@ slow_work_pending:
 	cmp	r0, #0
 	beq	no_work_pending
 	movlt	scno, #(__NR_restart_syscall - __NR_SYSCALL_BASE)
+	str	scno, [tsk, #TI_ABI_SYSCALL]	@ make sure tracers see update
 	ldmia	sp, {r0 - r6}			@ have to reload r0 - r6
 	b	local_restart			@ ... and off we go
 ENDPROC(ret_fast_syscall)
-- 
2.34.1


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[PATCH v2 2/2] ARM: ptrace: Restore syscall skipping for tracers

[Kees Cook]

Since commit 4e57a4ddf6b0 ("ARM: 9107/1: syscall: always store
thread_info->abi_syscall"), the seccomp selftests "syscall_errno"
and "syscall_faked" have been broken. Both seccomp and PTRACE depend
on using the special value of "-1" for skipping syscalls. This value
wasn't working because it was getting masked by __NR_SYSCALL_MASK in
both PTRACE_SET_SYSCALL and get_syscall_nr().

Explicitly test for -1 in PTRACE_SET_SYSCALL and get_syscall_nr(),
leaving it exposed when present, allowing tracers to skip syscalls
again.

Cc: Russell King <linux@armlinux.org.uk>
Cc: Arnd Bergmann <arnd@kernel.org>
Cc: Lecopzer Chen <lecopzer.chen@mediatek.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: linux-arm-kernel@lists.infradead.org
Fixes: 4e57a4ddf6b0 ("ARM: 9107/1: syscall: always store thread_info->abi_syscall")
Signed-off-by: Kees Cook <keescook@chromium.org>
---
 arch/arm/include/asm/syscall.h | 3 +++
 arch/arm/kernel/ptrace.c       | 5 +++--
 2 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/arch/arm/include/asm/syscall.h b/arch/arm/include/asm/syscall.h
index dfeed440254a..fe4326d938c1 100644
--- a/arch/arm/include/asm/syscall.h
+++ b/arch/arm/include/asm/syscall.h
@@ -25,6 +25,9 @@ static inline int syscall_get_nr(struct task_struct *task,
 	if (IS_ENABLED(CONFIG_AEABI) && !IS_ENABLED(CONFIG_OABI_COMPAT))
 		return task_thread_info(task)->abi_syscall;
 
+	if (task_thread_info(task)->abi_syscall == -1)
+		return -1;
+
 	return task_thread_info(task)->abi_syscall & __NR_SYSCALL_MASK;
 }
 
diff --git a/arch/arm/kernel/ptrace.c b/arch/arm/kernel/ptrace.c
index 2d8e2516906b..fef32d73f912 100644
--- a/arch/arm/kernel/ptrace.c
+++ b/arch/arm/kernel/ptrace.c
@@ -783,8 +783,9 @@ long arch_ptrace(struct task_struct *child, long request,
 			break;
 
 		case PTRACE_SET_SYSCALL:
-			task_thread_info(child)->abi_syscall = data &
-							__NR_SYSCALL_MASK;
+			if (data != -1)
+				data &= __NR_SYSCALL_MASK;
+			task_thread_info(child)->abi_syscall = data;
 			ret = 0;
 			break;
 
-- 
2.34.1


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Re: [PATCH v2 2/2] ARM: ptrace: Restore syscall skipping for tracers

[Arnd Bergmann]

On Thu, Aug 10, 2023, at 21:54, Kees Cook wrote:
> Since commit 4e57a4ddf6b0 ("ARM: 9107/1: syscall: always store
> thread_info->abi_syscall"), the seccomp selftests "syscall_errno"
> and "syscall_faked" have been broken. Both seccomp and PTRACE depend
> on using the special value of "-1" for skipping syscalls. This value
> wasn't working because it was getting masked by __NR_SYSCALL_MASK in
> both PTRACE_SET_SYSCALL and get_syscall_nr().
>
> Explicitly test for -1 in PTRACE_SET_SYSCALL and get_syscall_nr(),
> leaving it exposed when present, allowing tracers to skip syscalls
> again.
>
> Cc: Russell King <linux@armlinux.org.uk>
> Cc: Arnd Bergmann <arnd@kernel.org>
> Cc: Lecopzer Chen <lecopzer.chen@mediatek.com>
> Cc: Oleg Nesterov <oleg@redhat.com>
> Cc: linux-arm-kernel@lists.infradead.org
> Fixes: 4e57a4ddf6b0 ("ARM: 9107/1: syscall: always store 
> thread_info->abi_syscall")
> Signed-off-by: Kees Cook <keescook@chromium.org>

Reviewed-by: Arnd Bergmann <arnd@arndb.de>

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Re: [PATCH v2 1/2] ARM: ptrace: Restore syscall restart tracing

[Arnd Bergmann]

On Thu, Aug 10, 2023, at 21:54, Kees Cook wrote:
> Since commit 4e57a4ddf6b0 ("ARM: 9107/1: syscall: always store
> thread_info->abi_syscall"), the seccomp selftests "syscall_restart" has
> been broken. This was caused by the restart syscall not being stored to
> "abi_syscall" during restart setup before branching to the "local_restart"
> label. Tracers would see the wrong syscall, and scno would get overwritten
> while returning from the TIF_WORK path. Add the missing store.
>

> diff --git a/arch/arm/kernel/entry-common.S b/arch/arm/kernel/entry-common.S
> index bcc4c9ec3aa4..5c31e9de7a60 100644
> --- a/arch/arm/kernel/entry-common.S
> +++ b/arch/arm/kernel/entry-common.S
> @@ -90,6 +90,7 @@ slow_work_pending:
>  	cmp	r0, #0
>  	beq	no_work_pending
>  	movlt	scno, #(__NR_restart_syscall - __NR_SYSCALL_BASE)
> +	str	scno, [tsk, #TI_ABI_SYSCALL]	@ make sure tracers see update
>  	ldmia	sp, {r0 - r6}			@ have to reload r0 - r6
>  	b	local_restart			@ ... and off we go
>  ENDPROC(ret_fast_syscall)

Looks good to me, as in my previous reply this is safe for all cases
with the current restartable syscalls, though it would be wrong if
we had a restartblock version of epoll_wait on an OABI_COMPAT task,
which needs to look at the high bits of the number to tell which
structure layout to use.

I also looked at my original patch now and see what I did wrong
there.

Reviewed-by: Arnd Bergmann <arnd@arndb.de>

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Re: [PATCH v2 0/2] ARM: ptrace: Restore syscall skipping and restart while tracing

[Kees Cook]

On Thu, Aug 10, 2023 at 12:54:17PM -0700, Kees Cook wrote:
> Fix tracing on arm since commit 4e57a4ddf6b0 ("ARM: 9107/1: syscall:
> always store thread_info->abi_syscall"). This was seen with the broken
> seccomp tests "syscall_errno", "syscall_faked", and "syscall_restart".

I'd like to get this fixed in v6.5. Russell, do you have a fixes tree
that you'll be sending Linus's way before the next -rc? I can send it
via my seccomp-fixes tree if that'd be easier.

Thanks!

-Kees

-- 
Kees Cook

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Re: [PATCH v2 0/2] ARM: ptrace: Restore syscall skipping and restart while tracing

[Kees Cook]

On Thu, 10 Aug 2023 12:54:17 -0700, Kees Cook wrote:
> Fix tracing on arm since commit 4e57a4ddf6b0 ("ARM: 9107/1: syscall:
> always store thread_info->abi_syscall"). This was seen with the broken
> seccomp tests "syscall_errno", "syscall_faked", and "syscall_restart".

Applied to for-next/seccomp, thanks!

[1/2] ARM: ptrace: Restore syscall restart tracing
      https://git.kernel.org/kees/c/cf007647475b
[2/2] ARM: ptrace: Restore syscall skipping for tracers
      https://git.kernel.org/kees/c/4697b5848bd9

Take care,

-- 
Kees Cook


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