| Message ID | 2874581.1742399866@warthog.procyon.org.uk (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | [v2] keys: Fix UAF in key_put() | expand |
On 03/19, David Howells wrote: > > --- a/security/keys/gc.c > +++ b/security/keys/gc.c > @@ -218,8 +218,10 @@ static void key_garbage_collector(struct work_struct *work) > key = rb_entry(cursor, struct key, serial_node); > cursor = rb_next(cursor); > > - if (refcount_read(&key->usage) == 0) > + if (test_bit(KEY_FLAG_FINAL_PUT, &key->flags)) { > + smp_mb(); /* Clobber key->user after FINAL_PUT seen. */ > goto found_unreferenced_key; > + } > > if (unlikely(gc_state & KEY_GC_REAPING_DEAD_1)) { > if (key->type == key_gc_dead_keytype) { > diff --git a/security/keys/key.c b/security/keys/key.c > index 3d7d185019d3..7198cd2ac3a3 100644 > --- a/security/keys/key.c > +++ b/security/keys/key.c > @@ -658,6 +658,8 @@ void key_put(struct key *key) > key->user->qnbytes -= key->quotalen; > spin_unlock_irqrestore(&key->user->lock, flags); > } > + smp_mb(); /* key->user before FINAL_PUT set. */ Can't resist, smp_mb__before_atomic() should equally work, but this doesn't really matter, please forget. > + set_bit(KEY_FLAG_FINAL_PUT, &key->flags); > schedule_work(&key_gc_work); I believe this patch is correct, Reviewed-by: Oleg Nesterov <oleg@redhat.com>
On Wed, 19 Mar 2025 at 09:31, Oleg Nesterov <oleg@redhat.com> wrote: > > Can't resist, smp_mb__before_atomic() should equally work, > but this doesn't really matter, please forget. We really should have "test_bit_acquire()" and "set_bit_release()". Well, we do have the test_bit_acquire(). We just don't have the set_bit side, because we only have the bit clearing version (and it's called "clear_bit_unlock()" for historical reasons). Annoying. Linus
Linus Torvalds <torvalds@linuxfoundation.org> wrote:
> We really should have "test_bit_acquire()" and "set_bit_release()".
I considered using test_bit_acquire() but, as you say, there's no
set_bit_release() as yet. I could switch things to initialise the flag to set
on key creation and clear the flag instead.
David
On Wed, Mar 19, 2025 at 03:57:46PM +0000, David Howells wrote: > > Once a key's reference count has been reduced to 0, the garbage collector > thread may destroy it at any time and so key_put() is not allowed to touch > the key after that point. The most key_put() is normally allowed to do is > to touch key_gc_work as that's a static global variable. > > However, in an effort to speed up the reclamation of quota, this is now > done in key_put() once the key's usage is reduced to 0 - but now the code > is looking at the key after the deadline, which is forbidden. > > Fix this by using a flag to indicate that a key can be gc'd now rather than > looking at the key's refcount in the garbage collector. > > Fixes: 9578e327b2b4 ("keys: update key quotas in key_put()") > Reported-by: syzbot+6105ffc1ded71d194d6d@syzkaller.appspotmail.com > Signed-off-by: David Howells <dhowells@redhat.com> > Tested-by: syzbot+6105ffc1ded71d194d6d@syzkaller.appspotmail.com > cc: Jarkko Sakkinen <jarkko@kernel.org> > cc: Oleg Nesterov <oleg@redhat.com> > cc: Kees Cook <kees@kernel.org> > cc: Hillf Danton <hdanton@sina.com>, > cc: keyrings@vger.kernel.org > Cc: stable@vger.kernel.org # v6.10+ > --- > include/linux/key.h | 1 + > security/keys/gc.c | 4 +++- > security/keys/key.c | 2 ++ > 3 files changed, 6 insertions(+), 1 deletion(-) > > diff --git a/include/linux/key.h b/include/linux/key.h > index 074dca3222b9..ba05de8579ec 100644 > --- a/include/linux/key.h > +++ b/include/linux/key.h > @@ -236,6 +236,7 @@ struct key { > #define KEY_FLAG_ROOT_CAN_INVAL 7 /* set if key can be invalidated by root without permission */ > #define KEY_FLAG_KEEP 8 /* set if key should not be removed */ > #define KEY_FLAG_UID_KEYRING 9 /* set if key is a user or user session keyring */ > +#define KEY_FLAG_FINAL_PUT 10 /* set if final put has happened on key */ > > /* the key type and key description string > * - the desc is used to match a key against search criteria > diff --git a/security/keys/gc.c b/security/keys/gc.c > index 7d687b0962b1..f27223ea4578 100644 > --- a/security/keys/gc.c > +++ b/security/keys/gc.c > @@ -218,8 +218,10 @@ static void key_garbage_collector(struct work_struct *work) > key = rb_entry(cursor, struct key, serial_node); > cursor = rb_next(cursor); > > - if (refcount_read(&key->usage) == 0) > + if (test_bit(KEY_FLAG_FINAL_PUT, &key->flags)) { > + smp_mb(); /* Clobber key->user after FINAL_PUT seen. */ test_bit() is already atomic. https://docs.kernel.org/core-api/wrappers/atomic_bitops.html > goto found_unreferenced_key; > + } > > if (unlikely(gc_state & KEY_GC_REAPING_DEAD_1)) { > if (key->type == key_gc_dead_keytype) { > diff --git a/security/keys/key.c b/security/keys/key.c > index 3d7d185019d3..7198cd2ac3a3 100644 > --- a/security/keys/key.c > +++ b/security/keys/key.c > @@ -658,6 +658,8 @@ void key_put(struct key *key) > key->user->qnbytes -= key->quotalen; > spin_unlock_irqrestore(&key->user->lock, flags); > } > + smp_mb(); /* key->user before FINAL_PUT set. */ > + set_bit(KEY_FLAG_FINAL_PUT, &key->flags); Ditto. Nit: I'm just thinking should the name imply more like that "now key_put() is actually done". E.g., even something like KEY_FLAG_PUT_DONE would be more self-descriptive. > schedule_work(&key_gc_work); > } > } > > BR, Jarkko
Jarkko Sakkinen <jarkko@kernel.org> wrote: > > + if (test_bit(KEY_FLAG_FINAL_PUT, &key->flags)) { > > + smp_mb(); /* Clobber key->user after FINAL_PUT seen. */ > > test_bit() is already atomic. Atomiticity doesn't apply to test_bit() - it only matters when it does two (or more) accesses that must be perceptually indivisible (e.g. set_bit doing RMW). But atomiticity isn't the issue here, hence the barrier. You need to be looking at memory-barriers.txt, not atomic_bitops.txt. We have two things to correctly order and set_bit() does not imply sufficient barriering; test_and_set_bit() does, but not set_bit(), hence Linus's comment about really wanting a set_bit_release(). > > + smp_mb(); /* key->user before FINAL_PUT set. */ > > + set_bit(KEY_FLAG_FINAL_PUT, &key->flags); > > Ditto. Ditto. ;-) > Nit: I'm just thinking should the name imply more like that "now > key_put() is actually done". E.g., even something like KEY_FLAG_PUT_DONE > would be more self-descriptive. KEY_FLAG_PUT_DONE isn't right. There can be lots of puts on a single key - only the one that reduces it to 0 matters for this. You could call it KEY_FLAG_CAN_NOW_GC or KEY_FLAG_GC_ABLE. David
On Thu, Mar 20, 2025 at 04:39:11PM +0000, David Howells wrote: > Jarkko Sakkinen <jarkko@kernel.org> wrote: > > > > + if (test_bit(KEY_FLAG_FINAL_PUT, &key->flags)) { > > > + smp_mb(); /* Clobber key->user after FINAL_PUT seen. */ > > > > test_bit() is already atomic. > > Atomiticity doesn't apply to test_bit() - it only matters when it does two (or > more) accesses that must be perceptually indivisible (e.g. set_bit doing RMW). > > But atomiticity isn't the issue here, hence the barrier. You need to be > looking at memory-barriers.txt, not atomic_bitops.txt. > > We have two things to correctly order and set_bit() does not imply sufficient > barriering; test_and_set_bit() does, but not set_bit(), hence Linus's comment > about really wanting a set_bit_release(). Oops, I was hallucinating here. And yeah, test_and_set_bit() does imply full mb as you said. I was somehow remembering what I did in SGX driver incorrectly and that led me into misconclusions, sorry. if (test_and_set_bit(SGX_ENCL_IOCTL, &encl->flags)) return -EBUSY; > > > > + smp_mb(); /* key->user before FINAL_PUT set. */ > > > + set_bit(KEY_FLAG_FINAL_PUT, &key->flags); > > > > Ditto. > > Ditto. ;-) Duh, no need poke with the stick further (or deeper) ;-) > > > Nit: I'm just thinking should the name imply more like that "now > > key_put() is actually done". E.g., even something like KEY_FLAG_PUT_DONE > > would be more self-descriptive. > > KEY_FLAG_PUT_DONE isn't right. There can be lots of puts on a single key - > only the one that reduces it to 0 matters for this. You could call it > KEY_FLAG_CAN_NOW_GC or KEY_FLAG_GC_ABLE. Well all alternatives are fine but my thinking was that one that finally zeros the refcount, "finalizes put" (pick whatever you want anyway). > > David BR, Jarkko
On Thu, Mar 20, 2025 at 07:28:36PM +0200, Jarkko Sakkinen wrote: > On Thu, Mar 20, 2025 at 04:39:11PM +0000, David Howells wrote: > > Jarkko Sakkinen <jarkko@kernel.org> wrote: > > > > > > + if (test_bit(KEY_FLAG_FINAL_PUT, &key->flags)) { > > > > + smp_mb(); /* Clobber key->user after FINAL_PUT seen. */ > > > > > > test_bit() is already atomic. > > > > Atomiticity doesn't apply to test_bit() - it only matters when it does two (or > > more) accesses that must be perceptually indivisible (e.g. set_bit doing RMW). > > > > But atomiticity isn't the issue here, hence the barrier. You need to be > > looking at memory-barriers.txt, not atomic_bitops.txt. > > > > We have two things to correctly order and set_bit() does not imply sufficient > > barriering; test_and_set_bit() does, but not set_bit(), hence Linus's comment > > about really wanting a set_bit_release(). > > Oops, I was hallucinating here. And yeah, test_and_set_bit() does > imply full mb as you said. > > I was somehow remembering what I did in SGX driver incorrectly and > that led me into misconclusions, sorry. > > if (test_and_set_bit(SGX_ENCL_IOCTL, &encl->flags)) > return -EBUSY; > > > > > > > + smp_mb(); /* key->user before FINAL_PUT set. */ > > > > + set_bit(KEY_FLAG_FINAL_PUT, &key->flags); > > > > > > Ditto. > > > > Ditto. ;-) > > Duh, no need poke with the stick further (or deeper) ;-) > > > > > > Nit: I'm just thinking should the name imply more like that "now > > > key_put() is actually done". E.g., even something like KEY_FLAG_PUT_DONE > > > would be more self-descriptive. > > > > KEY_FLAG_PUT_DONE isn't right. There can be lots of puts on a single key - > > only the one that reduces it to 0 matters for this. You could call it > > KEY_FLAG_CAN_NOW_GC or KEY_FLAG_GC_ABLE. > > Well all alternatives are fine but my thinking was that one that finally > zeros the refcount, "finalizes put" (pick whatever you want anyway). I'll pick this one up tomorrow and put a PR out within this week. BR, Jarkko
On Thu, Mar 20, 2025 at 08:46:41PM +0200, Jarkko Sakkinen wrote: > On Thu, Mar 20, 2025 at 07:28:36PM +0200, Jarkko Sakkinen wrote: > > On Thu, Mar 20, 2025 at 04:39:11PM +0000, David Howells wrote: > > > Jarkko Sakkinen <jarkko@kernel.org> wrote: > > > > > > > > + if (test_bit(KEY_FLAG_FINAL_PUT, &key->flags)) { > > > > > + smp_mb(); /* Clobber key->user after FINAL_PUT seen. */ > > > > > > > > test_bit() is already atomic. > > > > > > Atomiticity doesn't apply to test_bit() - it only matters when it does two (or > > > more) accesses that must be perceptually indivisible (e.g. set_bit doing RMW). > > > > > > But atomiticity isn't the issue here, hence the barrier. You need to be > > > looking at memory-barriers.txt, not atomic_bitops.txt. > > > > > > We have two things to correctly order and set_bit() does not imply sufficient > > > barriering; test_and_set_bit() does, but not set_bit(), hence Linus's comment > > > about really wanting a set_bit_release(). > > > > Oops, I was hallucinating here. And yeah, test_and_set_bit() does > > imply full mb as you said. > > > > I was somehow remembering what I did in SGX driver incorrectly and > > that led me into misconclusions, sorry. > > > > if (test_and_set_bit(SGX_ENCL_IOCTL, &encl->flags)) > > return -EBUSY; > > > > > > > > > > + smp_mb(); /* key->user before FINAL_PUT set. */ > > > > > + set_bit(KEY_FLAG_FINAL_PUT, &key->flags); > > > > > > > > Ditto. > > > > > > Ditto. ;-) > > > > Duh, no need poke with the stick further (or deeper) ;-) > > > > > > > > > Nit: I'm just thinking should the name imply more like that "now > > > > key_put() is actually done". E.g., even something like KEY_FLAG_PUT_DONE > > > > would be more self-descriptive. > > > > > > KEY_FLAG_PUT_DONE isn't right. There can be lots of puts on a single key - > > > only the one that reduces it to 0 matters for this. You could call it > > > KEY_FLAG_CAN_NOW_GC or KEY_FLAG_GC_ABLE. > > > > Well all alternatives are fine but my thinking was that one that finally > > zeros the refcount, "finalizes put" (pick whatever you want anyway). > > I'll pick this one up tomorrow and put a PR out within this week. I can try get this done tomorrow fully (with only one patch in the PR) so that we would get it still to the ongoing release... BR, Jarkko
diff --git a/include/linux/key.h b/include/linux/key.h index 074dca3222b9..ba05de8579ec 100644 --- a/include/linux/key.h +++ b/include/linux/key.h @@ -236,6 +236,7 @@ struct key { #define KEY_FLAG_ROOT_CAN_INVAL 7 /* set if key can be invalidated by root without permission */ #define KEY_FLAG_KEEP 8 /* set if key should not be removed */ #define KEY_FLAG_UID_KEYRING 9 /* set if key is a user or user session keyring */ +#define KEY_FLAG_FINAL_PUT 10 /* set if final put has happened on key */ /* the key type and key description string * - the desc is used to match a key against search criteria diff --git a/security/keys/gc.c b/security/keys/gc.c index 7d687b0962b1..f27223ea4578 100644 --- a/security/keys/gc.c +++ b/security/keys/gc.c @@ -218,8 +218,10 @@ static void key_garbage_collector(struct work_struct *work) key = rb_entry(cursor, struct key, serial_node); cursor = rb_next(cursor); - if (refcount_read(&key->usage) == 0) + if (test_bit(KEY_FLAG_FINAL_PUT, &key->flags)) { + smp_mb(); /* Clobber key->user after FINAL_PUT seen. */ goto found_unreferenced_key; + } if (unlikely(gc_state & KEY_GC_REAPING_DEAD_1)) { if (key->type == key_gc_dead_keytype) { diff --git a/security/keys/key.c b/security/keys/key.c index 3d7d185019d3..7198cd2ac3a3 100644 --- a/security/keys/key.c +++ b/security/keys/key.c @@ -658,6 +658,8 @@ void key_put(struct key *key) key->user->qnbytes -= key->quotalen; spin_unlock_irqrestore(&key->user->lock, flags); } + smp_mb(); /* key->user before FINAL_PUT set. */ + set_bit(KEY_FLAG_FINAL_PUT, &key->flags); schedule_work(&key_gc_work); } }
Once a key's reference count has been reduced to 0, the garbage collector thread may destroy it at any time and so key_put() is not allowed to touch the key after that point. The most key_put() is normally allowed to do is to touch key_gc_work as that's a static global variable. However, in an effort to speed up the reclamation of quota, this is now done in key_put() once the key's usage is reduced to 0 - but now the code is looking at the key after the deadline, which is forbidden. Fix this by using a flag to indicate that a key can be gc'd now rather than looking at the key's refcount in the garbage collector. Fixes: 9578e327b2b4 ("keys: update key quotas in key_put()") Reported-by: syzbot+6105ffc1ded71d194d6d@syzkaller.appspotmail.com Signed-off-by: David Howells <dhowells@redhat.com> Tested-by: syzbot+6105ffc1ded71d194d6d@syzkaller.appspotmail.com cc: Jarkko Sakkinen <jarkko@kernel.org> cc: Oleg Nesterov <oleg@redhat.com> cc: Kees Cook <kees@kernel.org> cc: Hillf Danton <hdanton@sina.com>, cc: keyrings@vger.kernel.org Cc: stable@vger.kernel.org # v6.10+ --- include/linux/key.h | 1 + security/keys/gc.c | 4 +++- security/keys/key.c | 2 ++ 3 files changed, 6 insertions(+), 1 deletion(-)