| Message ID | 1503071821-13845-1-git-send-email-Dave.Martin@arm.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
On 18 August 2017 at 16:57, Dave Martin <Dave.Martin@arm.com> wrote: > There are some tricky dependencies between the different stages of > flushing the FPSIMD register state during exec, and these can race > with context switch in ways that can cause the old task's regs to > leak across. In particular, a context switch during the memset() can > cause some of the task's old FPSIMD registers to reappear. > > Disabling preemption for this small window would be no big deal for > performance: preemption is already disabled for similar scenarios > like updating the FPSIMD registers in sigreturn. > > So, instead of rearranging things in ways that might swap existing > subtle bugs for new ones, this patch just disables preemption > around the FPSIMD state flushing so that races of this type can't > occur here. This brings fpsimd_flush_thread() into line with other > code paths. > > Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org> > Cc: stable@vger.kernel.org > Fixes: 674c242c9323 ("arm64: flush FP/SIMD state correctly after execve()") > Signed-off-by: Dave Martin <Dave.Martin@arm.com> Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org> > --- > > NOTE TO MAINTAINERS: This is a fix, applicable to *v4.13* and stable. > > The kernel-mode NEON rework already queued for v4.14 turns this into a > local_bh_disable() as a side effect, so does not strictly need this > patch. When merging, the queued v4.14 changes should be retained. > > arch/arm64/kernel/fpsimd.c | 2 ++ > 1 file changed, 2 insertions(+) > > diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c > index 06da8ea..c7b4995 100644 > --- a/arch/arm64/kernel/fpsimd.c > +++ b/arch/arm64/kernel/fpsimd.c > @@ -161,9 +161,11 @@ void fpsimd_flush_thread(void) > { > if (!system_supports_fpsimd()) > return; > + preempt_disable(); > memset(¤t->thread.fpsimd_state, 0, sizeof(struct fpsimd_state)); > fpsimd_flush_task_state(current); > set_thread_flag(TIF_FOREIGN_FPSTATE); > + preempt_enable(); > } > > /* > -- > 2.1.4 >
diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c index 06da8ea..c7b4995 100644 --- a/arch/arm64/kernel/fpsimd.c +++ b/arch/arm64/kernel/fpsimd.c @@ -161,9 +161,11 @@ void fpsimd_flush_thread(void) { if (!system_supports_fpsimd()) return; + preempt_disable(); memset(¤t->thread.fpsimd_state, 0, sizeof(struct fpsimd_state)); fpsimd_flush_task_state(current); set_thread_flag(TIF_FOREIGN_FPSTATE); + preempt_enable(); } /*
There are some tricky dependencies between the different stages of flushing the FPSIMD register state during exec, and these can race with context switch in ways that can cause the old task's regs to leak across. In particular, a context switch during the memset() can cause some of the task's old FPSIMD registers to reappear. Disabling preemption for this small window would be no big deal for performance: preemption is already disabled for similar scenarios like updating the FPSIMD registers in sigreturn. So, instead of rearranging things in ways that might swap existing subtle bugs for new ones, this patch just disables preemption around the FPSIMD state flushing so that races of this type can't occur here. This brings fpsimd_flush_thread() into line with other code paths. Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org> Cc: stable@vger.kernel.org Fixes: 674c242c9323 ("arm64: flush FP/SIMD state correctly after execve()") Signed-off-by: Dave Martin <Dave.Martin@arm.com> --- NOTE TO MAINTAINERS: This is a fix, applicable to *v4.13* and stable. The kernel-mode NEON rework already queued for v4.14 turns this into a local_bh_disable() as a side effect, so does not strictly need this patch. When merging, the queued v4.14 changes should be retained. arch/arm64/kernel/fpsimd.c | 2 ++ 1 file changed, 2 insertions(+)