From fbd63c08cdcca5fb1315aca3172b3c9c272cfb4f Mon Sep 17 00:00:00 2001
From: Al Viro <viro@zeniv.linux.org.uk>
Date: Fri, 24 Sep 2021 00:35:42 +0000
Subject: [PATCH] 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
---
arch/csky/kernel/signal.c | 4 ++++
1 file changed, 4 insertions(+)
diff --git a/arch/csky/kernel/signal.c b/arch/csky/kernel/signal.c
index bc4238b9f709a..c7b763d2f526e 100644
--- 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);
--
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