[PATCH] arm64/fpsimd: ptrace: zero target's fpsimd_state, not the tracer's

[Breno Leitao]

sve_set_common() is the backend for PTRACE_SETREGSET(NT_ARM_SVE) and
PTRACE_SETREGSET(NT_ARM_SSVE). Every write in the function operates on
the tracee (target) - except a single memset that uses current instead,
zeroing the tracer's saved V0-V31 / FPSR / FPCR shadow on every ptrace
SETREGSET call.

The memset is meant to give the tracee a defined zero register image
before the user-supplied payload is copied in (for partial writes,
header-only writes, and FPSIMD<->SVE format switches). Aiming it at
current both denies the tracee that clean slate and silently corrupts
the tracer.

Due to FPSIMD lazy save/restore the wipe only takes effect when the
tracer's CPU FPSIMD binding is dropped after the memset; the next
return to userspace then reloads V0-V31, FPSR and FPCR as zero. No
signal is raised and ptrace() returns success.

Reproducible on an arm64 kernel with SVE: a single-threaded tracer that
loads a known pattern into V0-V31, issues PTRACE_SETREGSET(NT_ARM_SVE)
on a child, and reads V0-V31 back observes them all zeroed within tens
of thousands of iterations when a sibling thread keeps stealing the
FPSIMD CPU binding.

Fixes: 316283f276eb ("arm64/fpsimd: ptrace: Consistently handle partial writes to NT_ARM_(S)SVE")
Signed-off-by: Breno Leitao <leitao at debian.org>
---
 arch/arm64/kernel/ptrace.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c
index ba5eab23fd900..4d08598e2891d 100644
--- a/arch/arm64/kernel/ptrace.c
+++ b/arch/arm64/kernel/ptrace.c
@@ -983,8 +983,8 @@ static int sve_set_common(struct task_struct *target,
 	}
 
 	/* Always zero V regs, FPSR, and FPCR */
-	memset(&current->thread.uw.fpsimd_state, 0,
-	       sizeof(current->thread.uw.fpsimd_state));
+	memset(&target->thread.uw.fpsimd_state, 0,
+	       sizeof(target->thread.uw.fpsimd_state));
 
 	/* Registers: FPSIMD-only case */
 

---
base-commit: 9d0d467c3572e93c5faa2e5906a8bbcd70b24efd
change-id: 20260505-fix_ptrace-1bcad595c09e

Best regards,
--  
Breno Leitao <leitao at debian.org>