[PATCH 5.5 236/257] powerpc/64/tm: Dont let userspace set regs->trap via sigreturn

[Greg Kroah-Hartman]

From: Michael Ellerman <mpe@xxxxxxxxxxxxxx>

commit c7def7fbdeaa25feaa19caf4a27c5d10bd8789e4 upstream.

In restore_tm_sigcontexts() we take the trap value directly from the
user sigcontext with no checking:

	err |= __get_user(regs->trap, &sc->gp_regs[PT_TRAP]);

This means we can be in the kernel with an arbitrary regs->trap value.

Although that's not immediately problematic, there is a risk we could
trigger one of the uses of CHECK_FULL_REGS():

	#define CHECK_FULL_REGS(regs)	BUG_ON(regs->trap & 1)

It can also cause us to unnecessarily save non-volatile GPRs again in
save_nvgprs(), which shouldn't be problematic but is still wrong.

It's also possible it could trick the syscall restart machinery, which
relies on regs->trap not being == 0xc00 (see 9a81c16b5275 ("powerpc:
fix double syscall restarts")), though I haven't been able to make
that happen.

Finally it doesn't match the behaviour of the non-TM case, in
restore_sigcontext() which zeroes regs->trap.

So change restore_tm_sigcontexts() to zero regs->trap.

This was discovered while testing Nick's upcoming rewrite of the
syscall entry path. In that series the call to save_nvgprs() prior to
signal handling (do_notify_resume()) is removed, which leaves the
low-bit of regs->trap uncleared which can then trigger the FULL_REGS()
WARNs in setup_tm_sigcontexts().

Fixes: 2b0a576d15e0 ("powerpc: Add new transactional memory state to the signal context")
Cc: stable@xxxxxxxxxxxxxxx # v3.9+
Signed-off-by: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
Link: https://lore.kernel.org/r/20200401023836.3286664-1-mpe@xxxxxxxxxxxxxx
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 arch/powerpc/kernel/signal_64.c |    4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

--- a/arch/powerpc/kernel/signal_64.c
+++ b/arch/powerpc/kernel/signal_64.c
@@ -473,8 +473,10 @@ static long restore_tm_sigcontexts(struc
 	err |= __get_user(tsk->thread.ckpt_regs.ccr,
 			  &sc->gp_regs[PT_CCR]);
 
+	/* Don't allow userspace to set the trap value */
+	regs->trap = 0;
+
 	/* These regs are not checkpointed; they can go in 'regs'. */
-	err |= __get_user(regs->trap, &sc->gp_regs[PT_TRAP]);
 	err |= __get_user(regs->dar, &sc->gp_regs[PT_DAR]);
 	err |= __get_user(regs->dsisr, &sc->gp_regs[PT_DSISR]);
 	err |= __get_user(regs->result, &sc->gp_regs[PT_RESULT]);