[PATCH 4/5] exec: binfmt_misc: fix race between load_misc_binary() and kill_node()

From: Oleg Nesterov
Date: Fri Sep 22 2017 - 10:37:03 EST


load_misc_binary() makes a local copy of fmt->interpreter under entries_lock
to avoid the race with kill_node() but this is not enough; the whole Node
can be freed after we drop entries_lock, not only the ->interpreter string.

Add dget/dput(fmt->dentry) to ensure bm_evict_inode() can't destroy/free
this Node.

Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx>
---
fs/binfmt_misc.c | 12 ++++++++----
1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/fs/binfmt_misc.c b/fs/binfmt_misc.c
index 45809c6..e59d006 100644
--- a/fs/binfmt_misc.c
+++ b/fs/binfmt_misc.c
@@ -138,20 +138,23 @@ static int load_misc_binary(struct linux_binprm *bprm)

retval = -ENOEXEC;
if (!enabled)
- goto ret;
+ return retval;

/* to keep locking time low, we copy the interpreter string */
read_lock(&entries_lock);
fmt = check_file(bprm);
- if (fmt)
+ if (fmt) {
+ dget(fmt->dentry);
strlcpy(iname, fmt->interpreter, BINPRM_BUF_SIZE);
+ }
read_unlock(&entries_lock);
if (!fmt)
- goto ret;
+ return retval;

/* Need to be able to load the file after exec */
+ retval = -ENOENT;
if (bprm->interp_flags & BINPRM_FLAGS_PATH_INACCESSIBLE)
- return -ENOENT;
+ goto ret;

if (!(fmt->flags & MISC_FMT_PRESERVE_ARGV0)) {
retval = remove_arg_zero(bprm);
@@ -235,6 +238,7 @@ static int load_misc_binary(struct linux_binprm *bprm)
goto error;

ret:
+ dput(fmt->dentry);
return retval;
error:
if (fd_binary > 0)
--
2.5.0