2024-01-15 18:42:50

by Mark Brown

Subject: [PATCH] arm64/ptrace: Don't flush ZA/ZT storage when writing ZA via ptrace

When writing ZA we currently unconditionally flush the buffer used to store
it as part of ensuring that it is allocated. Since this buffer is shared
with ZT0 this means that a write to ZA when PSTATE.ZA is already set will
corrupt the value of ZT0 on a SME2 system. Fix this by only flushing the
backing storage if PSTATE.ZA was not previously set.

This will mean that short or failed writes may leave stale data in the
buffer, this seems as correct as our current behaviour and unlikely to be
something that userspace will rely on.

Fixes: f90b529bcbe5 ("arm64/sme: Implement ZT0 ptrace support")
Signed-off-by: Mark Brown <[email protected]>
---
arch/arm64/kernel/ptrace.c | 13 +++++++------
1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c
index 20d7ef82de90..b3f64144b5cd 100644
--- a/arch/arm64/kernel/ptrace.c
+++ b/arch/arm64/kernel/ptrace.c
@@ -1107,12 +1107,13 @@ static int za_set(struct task_struct *target,
}
}

- /* Allocate/reinit ZA storage */
- sme_alloc(target, true);
- if (!target->thread.sme_state) {
- ret = -ENOMEM;
- goto out;
- }
+ /*
+ * Only flush the storage if PSTATE.ZA was not already set,
+ * otherwise preserve any existing data.
+ */
+ sme_alloc(target, !thread_za_enabled(&target->thread));
+ if (!target->thread.sme_state)
+ return -ENOMEM;

/* If there is no data then disable ZA */
if (!count) {

---
base-commit: 0dd3ee31125508cd67f7e7172247f05b7fd1753a
change-id: 20240105-arm64-fix-ptrace-za-zt-5f64a61fd612

Best regards,
--
Mark Brown <[email protected]>

2024-01-18 12:02:52

by Will Deacon

Subject: Re: [PATCH] arm64/ptrace: Don't flush ZA/ZT storage when writing ZA via ptrace

On Mon, 15 Jan 2024 18:42:38 +0000, Mark Brown wrote:
> When writing ZA we currently unconditionally flush the buffer used to store
> it as part of ensuring that it is allocated. Since this buffer is shared
> with ZT0 this means that a write to ZA when PSTATE.ZA is already set will
> corrupt the value of ZT0 on a SME2 system. Fix this by only flushing the
> backing storage if PSTATE.ZA was not previously set.
>
> This will mean that short or failed writes may leave stale data in the
> buffer, this seems as correct as our current behaviour and unlikely to be
> something that userspace will rely on.
>
> [...]

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

[1/1] arm64/ptrace: Don't flush ZA/ZT storage when writing ZA via ptrace
https://git.kernel.org/arm64/c/b7c510d04904

Cheers,
--
Will

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