[PATCH V2] exportfs: do not read dentry after free

[PATCH V2] exportfs: do not read dentry after free

[Pan Bian]

The function dentry_connected calls dput(dentry) to drop the previously
acquired reference to dentry. In this case, dentry can be released.
After that, IS_ROOT(dentry) checks the condition
(dentry == dentry->d_parent), which may result in a use-after-free bug.
This patch directly compares dentry with its parent obtained before
dropping the reference.

Fixes: a056cc8934c("exportfs: stop retrying once we race with
rename/remove")

Signed-off-by: Pan Bian <bianpan2016@163.com>

---
V2: get rid of the comment

---
 fs/exportfs/expfs.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/exportfs/expfs.c b/fs/exportfs/expfs.c
index 645158d..a69aaf5 100644
--- a/fs/exportfs/expfs.c
+++ b/fs/exportfs/expfs.c
@@ -77,7 +77,7 @@ static bool dentry_connected(struct dentry *dentry)
 		struct dentry *parent = dget_parent(dentry);
 
 		dput(dentry);
-		if (IS_ROOT(dentry)) {
+		if (dentry == parent) {
 			dput(parent);
 			return false;
 		}
-- 
2.7.4

Re: [PATCH V2] exportfs: do not read dentry after free

[Al Viro]

On Fri, Nov 23, 2018 at 03:56:33PM +0800, Pan Bian wrote:
> The function dentry_connected calls dput(dentry) to drop the previously
> acquired reference to dentry. In this case, dentry can be released.
> After that, IS_ROOT(dentry) checks the condition
> (dentry == dentry->d_parent), which may result in a use-after-free bug.
> This patch directly compares dentry with its parent obtained before
> dropping the reference.

It's a bit more subtle than the description implies (the race has
dentry connected during dget_parent() and the child we'd reached it
through moved elsewhere during the dput()), but you are right - the
race is there and that patch fixes it.

I wonder if we could avoid those dget_parent()/dput() completely -
looks like we might be able to with rcu_read_lock() and some
care.  OTOH, that's not going to be a hot path, anyway...

Applied.

Re: [PATCH V2] exportfs: do not read dentry after free

[J. Bruce Fields]

On Fri, Nov 23, 2018 at 03:56:33PM +0800, Pan Bian wrote:
> The function dentry_connected calls dput(dentry) to drop the previously
> acquired reference to dentry. In this case, dentry can be released.
> After that, IS_ROOT(dentry) checks the condition
> (dentry == dentry->d_parent), which may result in a use-after-free bug.
> This patch directly compares dentry with its parent obtained before
> dropping the reference.

Looks right to me, thanks.--b.

> 
> Fixes: a056cc8934c("exportfs: stop retrying once we race with
> rename/remove")
> 
> Signed-off-by: Pan Bian <bianpan2016@163.com>
> 
> ---
> V2: get rid of the comment
> 
> ---
>  fs/exportfs/expfs.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/exportfs/expfs.c b/fs/exportfs/expfs.c
> index 645158d..a69aaf5 100644
> --- a/fs/exportfs/expfs.c
> +++ b/fs/exportfs/expfs.c
> @@ -77,7 +77,7 @@ static bool dentry_connected(struct dentry *dentry)
>  		struct dentry *parent = dget_parent(dentry);
>  
>  		dput(dentry);
> -		if (IS_ROOT(dentry)) {
> +		if (dentry == parent) {
>  			dput(parent);
>  			return false;
>  		}
> -- 
> 2.7.4
>