[PATCH] send_sigio_to_task: sanitize the usage of fown->signum

Oleg Nesterov
2009-06-16T22:33:03Z

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>

--- a/fs/fcntl.c~signum 2009-06-16 21:41:18.000000000 +0200
+++ b/fs/fcntl.c 2009-06-17 00:11:38.000000000 +0200
@@ -426,14 +426,20 @@ static inline int sigio_perm(struct task
}

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
@@ -442,7 +448,7 @@ static void send_sigio_to_task(struct ta
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_*
@@ -454,7 +460,7 @@ static void send_sigio_to_task(struct ta
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: