Subject: Re: [PATCH] powerpc/64/tm: Don't let userspace set regs->trap via sigreturn

From: Michael Ellerman <[email-masked]>
Date: Wed, 1 Apr 2020 23:53:40 +1100 (AEDT)

On Wed, 2020-04-01 at 02:38:36 UTC, Michael Ellerman wrote:
> 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: [email-masked] # v3.9+
> Signed-off-by: Michael Ellerman <[email-masked]>

Applied to powerpc next.

https://git.kernel.org/powerpc/c/c7def7fbdeaa25feaa19caf4a27c5d10bd8789e4

cheers