[patch 18/26] POWERPC: Fix subtle FP state corruption bug in signal return on SMP

Greg KH
2007-07-31T04:37:53Z

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-stable review patch. If anyone has any objections, please let us know.

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This fixes a bug which can cause corruption of the floating-point state
on return from a signal handler. If we have a signal handler that has
used the floating-point registers, and it happens to context-switch to
another task while copying the interrupted floating-point state from the
user stack into the thread struct (e.g. because of a page fault, or
because it gets preempted), the context switch code will think that the
FP registers contain valid FP state that needs to be copied into the
thread_struct, and will thus overwrite the values that the signal return
code has put into the thread_struct.

This can occur because we clear the MSR bits that indicate the presence
of valid FP state after copying the state into the thread_struct. To fix
this we just move the clearing of the MSR bits to before the copy. A
similar potential problem also occurs with the Altivec state, and this
fixes that in the same way.

Signed-off-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Chris Wright <chrisw@sous-sol.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
---
arch/powerpc/kernel/signal_64.c | 10 +++++++---
1 file changed, 7 insertions(+), 3 deletions(-)

--- linux-2.6.21.6.orig/arch/powerpc/kernel/signal_64.c
+++ linux-2.6.21.6/arch/powerpc/kernel/signal_64.c
@@ -177,6 +177,13 @@ static long restore_sigcontext(struct pt
*/
discard_lazy_cpu_state();

+ /*
+ * Force reload of FP/VEC.
+ * This has to be done before copying stuff into current->thread.fpr/vr
+ * for the reasons explained in the previous comment.
+ */
+ regs->msr &= ~(MSR_FP | MSR_FE0 | MSR_FE1 | MSR_VEC);
+
err |= __copy_from_user(&current->thread.fpr, &sc->fp_regs, FP_REGS_SIZE);

#ifdef CONFIG_ALTIVEC
@@ -198,9 +205,6 @@ static long restore_sigcontext(struct pt
current->thread.vrsave = 0;
#endif /* CONFIG_ALTIVEC */

- /* Force reload of FP/VEC */
- regs->msr &= ~(MSR_FP | MSR_FE0 | MSR_FE1 | MSR_VEC);
-
return err;
}

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