[Bionic][PATCH 1/1] powerpc/tm: Fix endianness flip on trap

Joseph Salisbury joseph.salisbury at canonical.com
Wed Apr 11 15:59:00 UTC 2018

From: Gustavo Romero <gromero at linux.vnet.ibm.com>

BugLink: http://bugs.launchpad.net/bugs/1762928

Currently it's possible that a thread on PPC64 LE has its endianness
flipped inadvertently to Big-Endian resulting in a crash once the process
is back from the signal handler.

If giveup_all() is called when regs->msr has the bits MSR.FP and MSR.VEC
disabled (and hence MSR.VSX disabled too) it returns without calling
check_if_tm_restore_required() which copies regs->msr to ckpt_regs->msr if
the process caught a signal whilst in transactional mode. Then once in
setup_tm_sigcontexts() MSR from ckpt_regs.msr is used, but since
check_if_tm_restore_required() was not called previuosly, gp_regs[PT_MSR]
gets a copy of invalid MSR bits as MSR in ckpt_regs was not updated from
regs->msr and so is zeroed. Later when leaving the signal handler once in
sys_rt_sigreturn() the TS bits of gp_regs[PT_MSR] are checked to determine
if restore_tm_sigcontexts() must be called to pull in the correct MSR state
into the user context. Because TS bits are zeroed
restore_tm_sigcontexts() is never called and MSR restored from the user
context on returning from the signal handler has the MSR.LE (the endianness
bit) forced to zero (Big-Endian). That leads, for instance, to 'nop' being
treated as an illegal instruction in the following sequence:

	beq	1f
1:	nop

on PPC64 LE machines and the process dies just after returning from the
signal handler.

PPC64 BE is also affected but in a subtle way since forcing Big-Endian on
a BE machine does not change the endianness.

This commit fixes the issue described above by ensuring that once in
setup_tm_sigcontexts() the MSR used is from regs->msr instead of from
ckpt_regs->msr and by ensuring that we pull in only the MSR.FP, MSR.VEC,
and MSR.VSX bits from ckpt_regs->msr.

The fix was tested both on LE and BE machines and no regression regarding
the powerpc/tm selftests was observed.

Signed-off-by: Gustavo Romero <gromero at linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe at ellerman.id.au>
(cherry picked from commit 1c200e63d055ec0125e44a5e386b9b78aada7eb3)
Signed-off-by: Joseph Salisbury <joseph.salisbury at canonical.com>
 arch/powerpc/kernel/signal_64.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/kernel/signal_64.c b/arch/powerpc/kernel/signal_64.c
index 4b9ca35..b1b9962 100644
--- a/arch/powerpc/kernel/signal_64.c
+++ b/arch/powerpc/kernel/signal_64.c
@@ -207,7 +207,7 @@ static long setup_tm_sigcontexts(struct sigcontext __user *sc,
 	elf_vrreg_t __user *tm_v_regs = sigcontext_vmx_regs(tm_sc);
 	struct pt_regs *regs = tsk->thread.regs;
-	unsigned long msr = tsk->thread.ckpt_regs.msr;
+	unsigned long msr = tsk->thread.regs->msr;
 	long err = 0;
 	BUG_ON(tsk != current);
@@ -216,6 +216,12 @@ static long setup_tm_sigcontexts(struct sigcontext __user *sc,
+	/* Restore checkpointed FP, VEC, and VSX bits from ckpt_regs as
+	 * it contains the correct FP, VEC, VSX state after we treclaimed
+	 * the transaction and giveup_all() was called on reclaiming.
+	 */
+	msr |= tsk->thread.ckpt_regs.msr & (MSR_FP | MSR_VEC | MSR_VSX);
 	/* Remove TM bits from thread's MSR.  The MSR in the sigcontext
 	 * just indicates to userland that we were doing a transaction, but we
 	 * don't want to return in transactional state.  This also ensures

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