csky: don't let sigreturn play with priveleged bits of status register

csky restore_sigcontext() blindly overwrites regs->sr with the value
it finds in sigcontext.  Attacker can store whatever they want in there,
which includes things like S-bit.  Userland shouldn't be able to set
that, or anything other than C flag (bit 0).

Do the same thing other architectures with protected bits in flags
register do - preserve everything that shouldn't be settable in
user mode, picking the rest from the value saved is sigcontext.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Guo Ren <guoren@kernel.org>
Cc: stable@vger.kernel.org

diff --git a/arch/csky/kernel/signal.c b/arch/csky/kernel/signal.c
index bc4238b9f709a8304d1766f1d7e2d3b06d9ac53c..c7b763d2f526e661f16b5ad98c5c56bd4c7c257e 100644 (file)
--- a/arch/csky/kernel/signal.c
+++ b/arch/csky/kernel/signal.c
@@ -52,10 +52,14 @@ static long restore_sigcontext(struct pt_regs *regs,
        struct sigcontext __user *sc)
 {
        int err = 0;
+       unsigned long sr = regs->sr;
 
        /* sc_pt_regs is structured the same as the start of pt_regs */
        err |= __copy_from_user(regs, &sc->sc_pt_regs, sizeof(struct pt_regs));
 
+       /* BIT(0) of regs->sr is Condition Code/Carry bit */
+       regs->sr = (sr & ~1) | (regs->sr & 1);
+
        /* Restore the floating-point state. */
        err |= restore_fpu_state(sc);