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