[PATCH] powerpc: Fix sigset_t copy

[Finn Thain]

From: Christophe Leroy <christophe.leroy at csgroup.eu>

The conversion from __copy_from_user() to __get_user() introduced a
regression in __get_user_sigset() in v5.13. The bug was subsequently
copied and pasted in unsafe_get_user_sigset().

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 casting the __get_user() assignment lvalue to u64
so that the entire struct gets copied.

Cc: Christophe Leroy <christophe.leroy at csgroup.eu>
Cc: Christopher M. Riedl <cmr at bluescreens.de>
Link: https://lore.kernel.org/linuxppc-dev/FEtBUOuFPMN4zJy4bIOqz6C4xoliCbTxS7VtMKD6UZkbvEbycUceRgGAd7e9-trRdwVN3hWAbQi0qrNx8Zgn8niTQf2KPVdw-W35czDIaeQ=@protonmail.com/
Fixes: 887f3ceb51cd ("powerpc/signal32: Convert do_setcontext[_tm]() to user access block")
Fixes: d3ccc9781560 ("powerpc/signal: Use __get_user() to copy sigset_t")
Reported-and-tested-by: Stan Johnson <userm57 at yahoo.com>
Signed-off-by: Finn Thain <fthain at linux-m68k.org>
---
Christophe, I hope this change is the one you wanted to see upstream (?).
If it is acceptable please add your signed-off-by tag.
---
 arch/powerpc/kernel/signal.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/powerpc/kernel/signal.h b/arch/powerpc/kernel/signal.h
index 1f07317964e4..44e736b88e91 100644
--- a/arch/powerpc/kernel/signal.h
+++ b/arch/powerpc/kernel/signal.h
@@ -23,10 +23,10 @@ static inline int __get_user_sigset(sigset_t *dst, const sigset_t __user *src)
 {
 	BUILD_BUG_ON(sizeof(sigset_t) != sizeof(u64));
 
-	return __get_user(dst->sig[0], (u64 __user *)&src->sig[0]);
+	return __get_user(*(u64 *)&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)
+	unsafe_get_user(*(u64 *)&(dst)->sig[0], (u64 __user *)&(src)->sig[0], label)
 
 #ifdef CONFIG_VSX
 extern unsigned long copy_vsx_to_user(void __user *to,
-- 
2.26.3