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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
This commit is contained in:
Al Viro 2021-09-24 00:35:42 +00:00 committed by Guo Ren
parent 64570fbc14
commit fbd63c08cd
1 changed files with 4 additions and 0 deletions
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4
arch/csky/kernel/signal.c
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@ -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);