From: Michael Ellerman mpe@ellerman.id.au
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@vger.kernel.org # v3.9+ Signed-off-by: Michael Ellerman mpe@ellerman.id.au Link: https://lore.kernel.org/r/20200401023836.3286664-1-mpe@ellerman.id.au Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- 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 @@ -462,8 +462,10 @@ static long restore_tm_sigcontexts(struc err |= __get_user(current->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]);