[PATCH] coredump: fix va_list corruption

[Eric Dumazet]

From: Eric Dumazet <edumazet@xxxxxxxxxx>

A va_list needs to be copied in case it needs to be used twice.

Thanks to Hugh for debugging this issue, leading to various panics.

Tested:

lpq84:~# echo "|/foobar12345 %h %h %h %h %h %h %h %h %h %h %h %h %h %h %
h %h %h %h %h %h" >/proc/sys/kernel/core_pattern

'produce_core' is simply : main() { *(int *)0 = 1;}

lpq84:~# ./produce_core
Segmentation fault (core dumped)
lpq84:~# dmesg | tail -1
[  614.352947] Core dump to |/foobar12345 lpq84 lpq84 lpq84 lpq84 lpq84
lpq84 lpq84 lpq84 lpq84 lpq84 lpq84 lpq84 lpq84 lpq84 lpq84 lpq84 lpq84
lpq84 lpq84 (null) pipe failed

Notice the last argument was replaced by a NULL (we were lucky enough to
not crash, but do not try this on your production machine !)

After fix :

lpq83:~# echo "|/foobar12345 %h %h %h %h %h %h %h %h %h %h %h %h %h %h %
h %h %h %h %h %h" >/proc/sys/kernel/core_pattern
lpq83:~# ./produce_core
Segmentation fault
lpq83:~# dmesg | tail -1
[  740.800441] Core dump to |/foobar12345 lpq83 lpq83 lpq83 lpq83 lpq83
lpq83 lpq83 lpq83 lpq83 lpq83 lpq83 lpq83 lpq83 lpq83 lpq83 lpq83 lpq83
lpq83 lpq83 lpq83 pipe failed

Fixes: 5fe9d8ca21cc ("coredump: cn_vprintf() has no reason to call vsnprintf() twice")
Signed-off-by: Eric Dumazet <edumazet@xxxxxxxxxx>
Diagnosed-by: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
Cc: Neil Horman <nhorman@xxxxxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx # 3.11+
---

Note: This patch applies cleanly to 3.11+ but might need some
adjustments for older linux kernels...

 fs/coredump.c |    7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/fs/coredump.c b/fs/coredump.c
index e3ad709a4232..0b2528fb640e 100644
--- a/fs/coredump.c
+++ b/fs/coredump.c
@@ -73,10 +73,15 @@ static int expand_corename(struct core_name *cn, int size)
 static int cn_vprintf(struct core_name *cn, const char *fmt, va_list arg)
 {
 	int free, need;
+	va_list arg_copy;
 
 again:
 	free = cn->size - cn->used;
-	need = vsnprintf(cn->corename + cn->used, free, fmt, arg);
+
+	va_copy(arg_copy, arg);
+	need = vsnprintf(cn->corename + cn->used, free, fmt, arg_copy);
+	va_end(arg_copy);
+
 	if (need < free) {
 		cn->used += need;
 		return 0;


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