Re: [PATCH] powerpc/tm: Avoid possible userspace r1 corruption on reclaim

Michael Neuling [off-list ref] writes:

On Tue, 2018-09-25 at 22:00 +1000, Michael Ellerman wrote:

quoted

Michael Neuling [off-list ref] writes:

quoted

Current we store the userspace r1 to PACATMSCRATCH before finally
saving it to the thread struct.

In theory an exception could be taken here (like a machine check or
SLB miss) that could write PACATMSCRATCH and hence corrupt the
userspace r1. The SLB fault currently doesn't touch PACATMSCRATCH, but
others do.

We've never actually seen this happen but it's theoretically
possible. Either way, the code is fragile as it is.

This patch saves r1 to the kernel stack (which can't fault) before we
turn MSR[RI] back on. PACATMSCRATCH is still used but only with
MSR[RI] off. We then copy r1 from the kernel stack to the thread
struct once we have MSR[RI] back on.

Suggested-by: Breno Leitao <leitao@debian.org>
Signed-off-by: Michael Neuling <redacted>
---
 arch/powerpc/kernel/tm.S | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/kernel/tm.S b/arch/powerpc/kernel/tm.S
index 701b0f5b09..8207816a1e 100644
--- a/arch/powerpc/kernel/tm.S
+++ b/arch/powerpc/kernel/tm.S

@@ -178,6 +178,12 @@ _GLOBAL(tm_reclaim)
 
 	std	r11, GPR11(r1)			/* Temporary stash */
 
+	/* Move r1 to kernel stack in case PACATMSCRATCH is used once
+	 * we turn on RI
+	 */

I see we still need to send you to Comment Formatting Re-Education Camp.

The rest of that file has they style, so I'm just keeping with that.  I can
submit a patch later to fix them all up.

Sure, but let's not add new ones.

And yeah fixing them all up in another patch would be good.

cheers