[PATCH] arm64/ptrace: Don't clear calling process' TIF_SME on OOM

[PATCH] arm64/ptrace: Don't clear calling process' TIF_SME on OOM

[Mark Brown]

If allocating memory for the target SVE state in za_set() fails we clear
TIF_SME for the ptracing task which is obviously not correct.  If we are
here we know that the target task already had neither TIF_SVE nor
TIF_SME set since we only need to allocate if either the target had not
used either SVE or SME and had no need to allocate state before or we
just changed the vector length with vec_set_vector_length() which clears
TIF_ for us on allocation failure so just remove the clear entirely.

Reported-by: Wang ShaoBo <bobo.shaobowang@huawei.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
---
 arch/arm64/kernel/ptrace.c | 2 --
 1 file changed, 2 deletions(-)

diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c
index eb7c08dfb834..041d2ae5c30a 100644
--- a/arch/arm64/kernel/ptrace.c
+++ b/arch/arm64/kernel/ptrace.c
@@ -1084,7 +1084,6 @@ static int za_set(struct task_struct *target,
 	if (!target->thread.sve_state) {
 		sve_alloc(target, false);
 		if (!target->thread.sve_state) {
-			clear_thread_flag(TIF_SME);
 			ret = -ENOMEM;
 			goto out;
 		}
@@ -1094,7 +1093,6 @@ static int za_set(struct task_struct *target,
 	sme_alloc(target);
 	if (!target->thread.za_state) {
 		ret = -ENOMEM;
-		clear_tsk_thread_flag(target, TIF_SME);
 		goto out;
 	}
 
-- 
2.30.2


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Re: [PATCH] arm64/ptrace: Don't clear calling process' TIF_SME on OOM

[Will Deacon]

On Fri, Sep 02, 2022 at 02:28:02PM +0100, Mark Brown wrote:
> If allocating memory for the target SVE state in za_set() fails we clear
> TIF_SME for the ptracing task which is obviously not correct.  If we are
> here we know that the target task already had neither TIF_SVE nor
> TIF_SME set since we only need to allocate if either the target had not
> used either SVE or SME and had no need to allocate state before or we
> just changed the vector length with vec_set_vector_length() which clears
> TIF_ for us on allocation failure so just remove the clear entirely.
> 
> Reported-by: Wang ShaoBo <bobo.shaobowang@huawei.com>
> Signed-off-by: Mark Brown <broonie@kernel.org>
> ---
>  arch/arm64/kernel/ptrace.c | 2 --
>  1 file changed, 2 deletions(-)

Is this fixing a real (i.e. user-observable) issue, or is it removing some
redundant code? I'm afraid I can't tell from the commit message and I'm
trying to figure out whether we should be queueing it for -rc (and CC'ing
stable) or not.

Thanks,

Will

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Re: [PATCH] arm64/ptrace: Don't clear calling process' TIF_SME on OOM

[Mark Brown]

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On Tue, Sep 06, 2022 at 05:27:15PM +0100, Will Deacon wrote:
> On Fri, Sep 02, 2022 at 02:28:02PM +0100, Mark Brown wrote:
> > If allocating memory for the target SVE state in za_set() fails we clear
> > TIF_SME for the ptracing task which is obviously not correct.  If we are

> Is this fixing a real (i.e. user-observable) issue, or is it removing some
> redundant code? I'm afraid I can't tell from the commit message and I'm
> trying to figure out whether we should be queueing it for -rc (and CC'ing
> stable) or not.

Since we're clearing TIF_SME for the process doing the ptrace there's
the potential for causing an issue in that process where we trap a new
SME usage and flush the SME state in the process which is a data
corruption issue which would be observable.

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Re: [PATCH] arm64/ptrace: Don't clear calling process' TIF_SME on OOM

[Will Deacon]

On Fri, 2 Sep 2022 14:28:02 +0100, Mark Brown wrote:
> If allocating memory for the target SVE state in za_set() fails we clear
> TIF_SME for the ptracing task which is obviously not correct.  If we are
> here we know that the target task already had neither TIF_SVE nor
> TIF_SME set since we only need to allocate if either the target had not
> used either SVE or SME and had no need to allocate state before or we
> just changed the vector length with vec_set_vector_length() which clears
> TIF_ for us on allocation failure so just remove the clear entirely.
> 
> [...]

Applied to arm64 (for-next/fixes), thanks!

[1/1] arm64/ptrace: Don't clear calling process' TIF_SME on OOM
      https://git.kernel.org/arm64/c/d105d6920ec7

Cheers,
-- 
Will

https://fixes.arm64.dev
https://next.arm64.dev
https://will.arm64.dev

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