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Re: [PATCH] tests/9p: fix potential leak in v9fs_rreaddir()
From: |
Christian Schoenebeck |
Subject: |
Re: [PATCH] tests/9p: fix potential leak in v9fs_rreaddir() |
Date: |
Sat, 29 Apr 2023 15:20:12 +0200 |
On Saturday, April 29, 2023 2:04:30 PM CEST Greg Kurz wrote:
> Hi Christian !
Hi there, it's been a while! :)
> On Sat, 29 Apr 2023 11:25:33 +0200
> Christian Schoenebeck <qemu_oss@crudebyte.com> wrote:
>
> > Free allocated directory entries in v9fs_rreaddir() if argument
> > `entries` was passed as NULL, to avoid a memory leak. It is
> > explicitly allowed by design for `entries` to be NULL. [1]
> >
> > [1] https://lore.kernel.org/all/1690923.g4PEXVpXuU@silver
> >
> > Reported-by: Coverity (CID 1487558)
> > Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
> > ---
>
> Good catch Coverity ! :-)
Yeah, this Coverity report is actually from March and I ignored it so far,
because the reported leak could never happen with current test code. But Paolo
brought it up this week, so ...
> Reviewed-by: Greg Kurz <groug@kaod.org>
>
> I still have a suggestion. See below.
>
> > tests/qtest/libqos/virtio-9p-client.c | 5 +++++
> > 1 file changed, 5 insertions(+)
> >
> > diff --git a/tests/qtest/libqos/virtio-9p-client.c
> > b/tests/qtest/libqos/virtio-9p-client.c
> > index e4a368e036..b8adc8d4b9 100644
> > --- a/tests/qtest/libqos/virtio-9p-client.c
> > +++ b/tests/qtest/libqos/virtio-9p-client.c
> > @@ -594,6 +594,8 @@ void v9fs_rreaddir(P9Req *req, uint32_t *count,
> > uint32_t *nentries,
> > {
> > uint32_t local_count;
> > struct V9fsDirent *e = NULL;
> > + /* only used to avoid a leak if entries was NULL */
> > + struct V9fsDirent *unused_entries = NULL;
> > uint16_t slen;
> > uint32_t n = 0;
> >
> > @@ -612,6 +614,8 @@ void v9fs_rreaddir(P9Req *req, uint32_t *count,
> > uint32_t *nentries,
> > e = g_new(struct V9fsDirent, 1);
> > if (entries) {
> > *entries = e;
> > + } else {
> > + unused_entries = e;
> > }
> > } else {
> > e = e->next = g_new(struct V9fsDirent, 1);
>
> This is always allocating and chaining a new entry even
> though it isn't needed in the entries == NULL case.
>
> > @@ -628,6 +632,7 @@ void v9fs_rreaddir(P9Req *req, uint32_t *count,
> > uint32_t *nentries,
> > *nentries = n;
> > }
> >
> > + v9fs_free_dirents(unused_entries);
>
> This is going to loop again on all entries to free them.
>
> > v9fs_req_free(req);
> > }
> >
>
> If this function is to be called one day with an enormous
> number of entries and entries == NULL case, this might
> not scale well.
>
> What about only allocating a single entry in this case ?
>
> E.g.
>
> @@ -593,7 +593,7 @@ void v9fs_rreaddir(P9Req *req, uint32_t *count, uint32_t
> *nentries,
> struct V9fsDirent **entries)
> {
> uint32_t local_count;
> - struct V9fsDirent *e = NULL;
> + g_autofree struct V9fsDirent *e = NULL;
> uint16_t slen;
> uint32_t n = 0;
>
> @@ -611,10 +611,12 @@ void v9fs_rreaddir(P9Req *req, uint32_t *count,
> uint32_t *nentries,
> if (!e) {
> e = g_new(struct V9fsDirent, 1);
> if (entries) {
> - *entries = e;
> + *entries = g_steal_pointer(e);
g_steal_pointer(e) just sets `e` to NULL and returns its old value, so ...
> }
> } else {
> - e = e->next = g_new(struct V9fsDirent, 1);
> + if (entries) {
> + e = e->next = g_new(struct V9fsDirent, 1);
> + }
... this `else` block would never be reached and no list assembled.
> }
> e->next = NULL;
> /* qid[13] offset[8] type[1] name[s] */
And even if above's issue was fixed, then it would cause a use-after-free for
the last element in the list if entries != NULL and caller trying to access
the last element afterwards. So you would still need a separate g_autofree
pointer instead of tagging `e` directly, or something like this after loop
end:
if (entries)
g_steal_pointer(e);
Which would somehow defeat the purpose of using g_autofree though.
I mean, yes this could be addressed, but is it worth it? I don't know. Even
this reported leak is a purely theoretical one, but I understand if people
want to silence a warning.
Best regards,
Christian Schoenebeck