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:
on PPC64 LE machines and the process dies just after returning from the
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.
+ /* 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
On Wed, Apr 11, 2018 at 11:58:59AM -0400, Joseph Salisbury wrote:
> BugLink: http://bugs.launchpad.net/bugs/1762928 >
> == Bionic Justification ==
> IBM is requesting this commit in Bionic because the tm_trap_test failed on
> P8 compat mode. Guests on a P9 host running bionic final beta version.
> Commit 1c200e63d055 is in mailine as of v4.16-rc1.
> == Fix ==
> 1c200e63d055 ("powerpc/tm: Fix endianness flip on trap")