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Subject[PATCH] information leak in sigaltstack
Unfortunately the stack_t data structure was defined before people
cared much about 64-bit architectures. It has a hole in the middle.
And this hole is creating an information leak in the sigaltstack

When the second parameter to sigaltstack is non-NULL the current stack
information is passed to the caller by filling in the members of the
stack_t structure and then the whole structure is copied using
copy_to_user. This unfortunately leaves the whole after flags

The following patch should fix the issue.

Signed-off-by: Ulrich Drepper <>

diff --git a/kernel/signal.c b/kernel/signal.c
index ccf1cee..612d6b7 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -2455,6 +2455,9 @@ do_sigaltstack (const stack_t __user *uss, stack_t __user *uoss, unsigned long s
int error;

if (uoss) {
+ if (offsetof(stack_t, ss_flags) + sizeof(oss.ss_flags) !=
+ offsetof(stack_t, ss_size))
+ memset(&oss, '\0', sizeof(oss));
oss.ss_sp = (void __user *) current->sas_ss_sp;
oss.ss_size = current->sas_ss_size;
oss.ss_flags = sas_ss_flags(sp);

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