linux-kernel - [PATCH] powerpc/signal32: Fix sigset

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Date:   Mon, 15 Nov 2021 09:52:55 +0100
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>
Cc:     Christophe Leroy <christophe.leroy@...roup.eu>,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        "Christopher M . Riedl" <cmr@...escreens.de>,
        stable@...r.kernel.org, Finn Thain <fthain@...ux-m68k.org>,
        Stan Johnson <userm57@...oo.com>
Subject: [PATCH] powerpc/signal32: Fix sigset_t copy

The conversion from __copy_from_user() to __get_user() by
commit d3ccc9781560 ("powerpc/signal: Use __get_user() to copy
sigset_t") introduced a regression in __get_user_sigset() for
powerpc/32. The bug was subsequently moved into
unsafe_get_user_sigset().

The bug is due to the copied 64 bit value being truncated to
32 bits while being assigned to dst->sig[0]

The regression was reported by users of the Xorg packages distributed in
Debian/powerpc --

    "The symptoms are that the fb screen goes blank, with the backlight
    remaining on and no errors logged in /var/log; wdm (or startx) run
    with no effect (I tried logging in in the blind, with no effect).
    And they are hard to kill, requiring 'kill -KILL ...'"

Fix the regression by copying each word of the sigset, not only the
first one.

__get_user_sigset() was tentatively optimised to copy 64 bits at once
in order to minimise KUAP unlock/lock impact, but the unsafe variant
doesn't suffer that, so it can just copy words.

Cc: Christopher M. Riedl <cmr@...escreens.de>
Fixes: 887f3ceb51cd ("powerpc/signal32: Convert do_setcontext[_tm]() to user access block")
Cc: stable@...r.kernel.org
Reported-by: Finn Thain <fthain@...ux-m68k.org>
Reported-and-tested-by: Stan Johnson <userm57@...oo.com>
Signed-off-by: Christophe Leroy <christophe.leroy@...roup.eu>
---
 arch/powerpc/kernel/signal.h | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/arch/powerpc/kernel/signal.h b/arch/powerpc/kernel/signal.h
index 1f07317964e4..618aeccdf691 100644
--- a/arch/powerpc/kernel/signal.h
+++ b/arch/powerpc/kernel/signal.h
@@ -25,8 +25,14 @@ static inline int __get_user_sigset(sigset_t *dst, const sigset_t __user *src)
 
 	return __get_user(dst->sig[0], (u64 __user *)&src->sig[0]);
 }
-#define unsafe_get_user_sigset(dst, src, label) \
-	unsafe_get_user((dst)->sig[0], (u64 __user *)&(src)->sig[0], label)
+#define unsafe_get_user_sigset(dst, src, label) do {			\
+	sigset_t *__dst = dst;						\
+	const sigset_t __user *__src = src;				\
+	int i;								\
+									\
+	for (i = 0; i < _NSIG_WORDS; i++)				\
+		unsafe_get_user(__dst->sig[i], &__src->sig[i], label);	\
+} while (0)
 
 #ifdef CONFIG_VSX
 extern unsigned long copy_vsx_to_user(void __user *to,
-- 
2.31.1