[PATCH 5.15 208/279] powerpc/signal32: Fix sigset_t copy

[Greg Kroah-Hartman]

From: Christophe Leroy <christophe.leroy@xxxxxxxxxx>

commit 5499802b2284331788a440585869590f1bd63f7f upstream.

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.

Fixes: 887f3ceb51cd ("powerpc/signal32: Convert do_setcontext[_tm]() to user access block")
Cc: stable@xxxxxxxxxxxxxxx # v5.13+
Reported-by: Finn Thain <fthain@xxxxxxxxxxxxxx>
Reported-and-tested-by: Stan Johnson <userm57@xxxxxxxxx>
Signed-off-by: Christophe Leroy <christophe.leroy@xxxxxxxxxx>
Signed-off-by: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
Link: https://lore.kernel.org/r/99ef38d61c0eb3f79c68942deb0c35995a93a777.1636966353.git.christophe.leroy@xxxxxxxxxx
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 arch/powerpc/kernel/signal.h |   10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

--- a/arch/powerpc/kernel/signal.h
+++ b/arch/powerpc/kernel/signal.h
@@ -25,8 +25,14 @@ static inline int __get_user_sigset(sigs
 
 	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,