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Subject[PATCH] seccomp: fix UAF in user-trap code
Date
On the failure path, we do an fput() of the listener fd if the filter fails
to install (e.g. because of a TSYNC race that's lost, or if the thread is
killed, etc.). fput() doesn't actually release the fd, it just ads it to a
work queue. Then the thread proceeds to free the filter, even though the
listener struct file has a reference to it.

To fix this, on the failure path let's set the private data to null, so we
know in ->release() to ignore the filter.

Reported-by: syzbot+981c26489b2d1c6316ba@syzkaller.appspotmail.com
Fixes: 6a21cc50f0c7 ("seccomp: add a return code to trap to userspace")
Signed-off-by: Tycho Andersen <tycho@tycho.ws>
---
This is a little ugly, but I can't really think of a better way (other than
force freeing, but the fput function that does the actual work is declared
static with four underscores :).
---
kernel/seccomp.c | 4 ++++
1 file changed, 4 insertions(+)

diff --git a/kernel/seccomp.c b/kernel/seccomp.c
index d7f538847b84..e815781ed751 100644
--- a/kernel/seccomp.c
+++ b/kernel/seccomp.c
@@ -976,6 +976,9 @@ static int seccomp_notify_release(struct inode *inode, struct file *file)
struct seccomp_filter *filter = file->private_data;
struct seccomp_knotif *knotif;

+ if (!filter)
+ return 0;
+
mutex_lock(&filter->notify_lock);

/*
@@ -1300,6 +1303,7 @@ static long seccomp_set_mode_filter(unsigned int flags,
out_put_fd:
if (flags & SECCOMP_FILTER_FLAG_NEW_LISTENER) {
if (ret < 0) {
+ listener_f->private_data = NULL;
fput(listener_f);
put_unused_fd(listener);
} else {
--
2.19.1
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