send_sigio_to_task: sanitize the usage of fown->signum

send_sigio_to_task() reads fown->signum several times, we can race with
F_SETSIG which changes ->signum lockless.  In theory, this can fool
security checks or we can call group_send_sig_info() with the wrong
->si_signo which does not match "int sig".

Change the code to cache ->signum.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 11 insertions, 5 deletions

diff --git a/fs/fcntl.c b/fs/fcntl.c
index f9c03ca3b2f4..a040b764f8e3 100644
--- a/fs/fcntl.c
+++ b/fs/fcntl.c
@@ -428,14 +428,20 @@ static inline int sigio_perm(struct task_struct *p,
 }
 
 static void send_sigio_to_task(struct task_struct *p,
-                               struct fown_struct *fown, 
+                               struct fown_struct *fown,
                                int fd,
                                int reason)
 {
-        if (!sigio_perm(p, fown, fown->signum))
+        /*
+         * F_SETSIG can change ->signum lockless in parallel, make
+         * sure we read it once and use the same value throughout.
+         */
+        int signum = ACCESS_ONCE(fown->signum);
+
+        if (!sigio_perm(p, fown, signum))
                 return;
 
-        switch (fown->signum) {
+        switch (signum) {
                 siginfo_t si;
                 default:
                         /* Queue a rt signal with the appropriate fd as its
@@ -444,7 +450,7 @@ static void send_sigio_to_task(struct task_struct *p,
                            delivered even if we can't queue.  Failure to
                            queue in this case _should_ be reported; we fall
                            back to SIGIO in that case. --sct */
-                        si.si_signo = fown->signum;
+                        si.si_signo = signum;
                         si.si_errno = 0;
                         si.si_code  = reason;
                         /* Make sure we are called with one of the POLL_*
@@ -456,7 +462,7 @@ static void send_sigio_to_task(struct task_struct *p,
                         else
                                 si.si_band = band_table[reason - POLL_IN];
                         si.si_fd    = fd;
-                        if (!group_send_sig_info(fown->signum, &si, p))
+                        if (!group_send_sig_info(signum, &si, p))
                                 break;
                 /* fall-through: fall back on the old plain SIGIO signal */
                 case 0: