[PATCH] cifs.upcall: fix UAF in get_cachename_from_process_env()

[Paulo Alcantara <pc@manguebit.com>]

Whether lseek(2) fails or @bufsize * 2 > ENV_BUF_MAX, then @buf would
end up being freed twice.  For instance:

  cifs-utils-7.0/cifs.upcall.c:501: freed_arg: "free" frees "buf".
  cifs-utils-7.0/cifs.upcall.c:524: double_free: Calling "free" frees
  pointer "buf" which has already been freed.
    522|           }
    523|   out_close:
    524|->         free(buf);
    525|           close(fd);
    526|           return cachename;

Fix this by setting @buf to NULL after freeing it to prevent UAF.

Fixes: ed97e4ecab4e ("cifs.upcall: allow scraping of KRB5CCNAME out of initiating task's /proc/<pid>/environ file")
Signed-off-by: Paulo Alcantara (Red Hat) <pc@manguebit.com>
---
 cifs.upcall.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/cifs.upcall.c b/cifs.upcall.c
index 52c03280dbe0..ff6f2bd271bc 100644
--- a/cifs.upcall.c
+++ b/cifs.upcall.c
@@ -498,10 +498,11 @@ retry:
 		/* We read to the end of the buffer. Double and try again */
 		syslog(LOG_DEBUG, "%s: read to end of buffer (%zu bytes)\n",
 					__func__, bufsize);
-		free(buf);
-		bufsize *= 2;
 		if (lseek(fd, 0, SEEK_SET) < 0)
 			goto out_close;
+		free(buf);
+		buf = NULL;
+		bufsize *= 2;
 		goto retry;
 	}
 
-- 
2.44.0