From: Hui Zhu <[email protected]>
Two UAF scenarios exist in bpf_trampoline_multi_detach() error paths:
1. If __bpf_trampoline_unlink_prog() fails, cur_image == old_image and
ftrace still points to it, but bpf_trampoline_multi_attach_free()
unconditionally frees old_image. Fix: only free old_image when it
differs from cur_image.
2. If the batch update_ftrace_direct_del/mod() fails, ftrace still
points to old_image, but _free() frees it. Fix: use rollback
(restores cur_image = old_image) instead of _free() for affected
mnodes.
Rollback keeps the image alive but the caller still frees the prog
whose call is baked into it. Pin the prog on old_image via a new
pinned_prog field in struct bpf_tramp_image (released in
bpf_tramp_image_free()).
bpf_trampoline_put() must not free a trampoline whose cur_image was
left behind by rollback -- ftrace may still call into it. Leak the
trampoline instead (it's already unlinked from lookup tables).
pinned_prog is a single pointer: if multiple progs need pinning on the
same image (rare), only the last is tracked and earlier refs are leaked
(not a UAF). This is an intentional trade-off.
Also make bpf_trampoline_multi_detach() return void since callers
cannot usefully react to failures.
Fixes: aef4dfa790b2 ("bpf: Add bpf_trampoline_multi_attach/detach functions")
Signed-off-by: Hui Zhu <[email protected]>
---
include/linux/bpf.h | 20 +++++--
kernel/bpf/trampoline.c | 112 +++++++++++++++++++++++++++++++++++----
kernel/trace/bpf_trace.c | 2 +-
3 files changed, 118 insertions(+), 16 deletions(-)
diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 73bacfc6444d..cd32c6f54eeb 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -1372,6 +1372,17 @@ struct bpf_tramp_image {
struct rcu_head rcu;
struct work_struct work;
};
+ /*
+ * Extra reference on the bpf_prog whose call is baked into this
+ * image's machine code, held only when a required ftrace
+ * direct-call update failed while retiring/replacing this image
+ * (see bpf_trampoline_multi_attach()/_detach() in trampoline.c).
+ * ftrace may still be directing calls into this image, so neither
+ * the image nor the pinned prog can be freed until a later,
+ * successful ftrace update proves this image is no longer in use.
+ * Released in bpf_tramp_image_free() alongside the image itself.
+ */
+ struct bpf_prog *pinned_prog;
};
struct bpf_trampoline {
@@ -1518,8 +1529,8 @@ int arch_prepare_bpf_dispatcher(void *image, void *buf,
s64 *funcs, int num_func
int bpf_trampoline_multi_attach(struct bpf_prog *prog, u32 *ids,
struct bpf_tracing_multi_link *link);
-int bpf_trampoline_multi_detach(struct bpf_prog *prog,
- struct bpf_tracing_multi_link *link);
+void bpf_trampoline_multi_detach(struct bpf_prog *prog,
+ struct bpf_tracing_multi_link *link);
void bpf_trampoline_set_flags(struct bpf_trampoline *tr, u32 flags);
/*
@@ -1639,10 +1650,9 @@ static inline int bpf_trampoline_multi_attach(struct
bpf_prog *prog, u32 *ids,
{
return -ENOTSUPP;
}
-static inline int bpf_trampoline_multi_detach(struct bpf_prog *prog,
- struct bpf_tracing_multi_link
*link)
+static inline void bpf_trampoline_multi_detach(struct bpf_prog *prog,
+ struct bpf_tracing_multi_link
*link)
{
- return -ENOTSUPP;
}
static inline void bpf_trampoline_set_flags(struct bpf_trampoline *tr, u32
flags) {}
#endif
diff --git a/kernel/bpf/trampoline.c b/kernel/bpf/trampoline.c
index ed7999ad6c66..c08d1a09e638 100644
--- a/kernel/bpf/trampoline.c
+++ b/kernel/bpf/trampoline.c
@@ -535,6 +535,14 @@ static void bpf_tramp_image_free(struct bpf_tramp_image
*im)
arch_free_bpf_trampoline(im->image, im->size);
bpf_jit_uncharge_modmem(im->size);
percpu_ref_exit(&im->pcref);
+ /*
+ * This image is confirmed no longer reachable from ftrace (that's
+ * why we're freeing it), so it's now safe to drop the reference we
+ * pinned on its behalf while it may have still been live - see
+ * bpf_trampoline_multi_attach()/_detach().
+ */
+ if (im->pinned_prog)
+ bpf_prog_put(im->pinned_prog);
kfree_rcu(im, rcu);
}
@@ -1216,6 +1224,28 @@ void bpf_trampoline_put(struct bpf_trampoline *tr)
*/
hlist_del(&tr->hlist_key);
hlist_del(&tr->hlist_ip);
+
+ /*
+ * tr->cur_image should already be NULL here. A non-NULL value means
+ * bpf_trampoline_multi_attach_rollback() left an image behind
+ * because a required ftrace direct-call update failed (see
+ * bpf_trampoline_multi_detach()), so ftrace may still be calling
+ * into it - and, in turn, into the bpf_prog pinned in
+ * tr->cur_image->pinned_prog. We have no reliable way to confirm
+ * ftrace has since stopped referencing it, so freeing
+ * tr->cur_image (and dropping the pinned prog's reference) here
+ * would risk a use-after-free.
+ *
+ * tr has just been unlinked from the lookup tables above, so any
+ * future attach to this function allocates a fresh trampoline;
+ * this one, its stuck image, and the pinned prog reference are
+ * deliberately leaked instead of freed. This is rare (it only
+ * happens after a genuine ftrace direct-call update failure) and
+ * bounded (at most one image), so it is far preferable to a UAF.
+ */
+ if (WARN_ON_ONCE(tr->cur_image))
+ goto out;
+
direct_ops_free(tr);
kfree(tr);
out:
@@ -1595,7 +1625,18 @@ static void bpf_trampoline_multi_attach_init(struct
bpf_trampoline *tr)
static void bpf_trampoline_multi_attach_free(struct bpf_trampoline *tr)
{
- if (tr->multi_attach.old_image)
+ /*
+ * Only free old_image if it is no longer the active image.
+ * When bpf_trampoline_update() fails before modify_fentry_multi()/
+ * unregister_fentry_multi() is called, cur_image is unchanged
+ * (cur_image == old_image) and ftrace still points to it. Freeing
+ * it would cause a UAF when ftrace calls into the freed memory.
+ * On success, cur_image is either a new image or NULL, so
+ * old_image != cur_image correctly identifies a stale image that
+ * is safe to free.
+ */
+ if (tr->multi_attach.old_image &&
+ tr->multi_attach.old_image != tr->cur_image)
bpf_tramp_image_put(tr->multi_attach.old_image);
tr->multi_attach.old_image = NULL;
@@ -1719,11 +1760,11 @@ int bpf_trampoline_multi_attach(struct bpf_prog *prog,
u32 *ids,
return err;
}
-int bpf_trampoline_multi_detach(struct bpf_prog *prog, struct
bpf_tracing_multi_link *link)
+void bpf_trampoline_multi_detach(struct bpf_prog *prog, struct
bpf_tracing_multi_link *link)
{
struct bpf_tracing_multi_data *data = &link->data;
struct bpf_tracing_multi_node *mnode;
- int i, err;
+ int i, err, err_unreg = 0, err_mod = 0;
trampoline_lock_all();
@@ -1735,13 +1776,65 @@ int bpf_trampoline_multi_detach(struct bpf_prog *prog,
struct bpf_tracing_multi_
WARN_ONCE(err, "__bpf_trampoline_unlink_prog failed: %d\n",
err);
}
- if (ftrace_hash_count(data->unreg))
- WARN_ON_ONCE(update_ftrace_direct_del(&direct_ops,
data->unreg));
- if (ftrace_hash_count(data->modify))
- WARN_ON_ONCE(update_ftrace_direct_mod(&direct_ops,
data->modify, true));
+ if (ftrace_hash_count(data->unreg)) {
+ err_unreg = update_ftrace_direct_del(&direct_ops, data->unreg);
+ WARN_ON_ONCE(err_unreg);
+ }
+ if (ftrace_hash_count(data->modify)) {
+ err_mod = update_ftrace_direct_mod(&direct_ops, data->modify,
true);
+ WARN_ON_ONCE(err_mod);
+ }
- for_each_mnode(mnode, link)
- bpf_trampoline_multi_attach_free(mnode->trampoline);
+ for_each_mnode(mnode, link) {
+ struct bpf_trampoline *tr = mnode->trampoline;
+
+ /* If the batch ftrace update failed for this mnode's path,
+ * ftrace still points to old_image. Use rollback to restore
+ * cur_image to old_image (putting the new cur_image if any)
+ * so the trampoline keeps the image ftrace is calling.
+ *
+ * A link only reaches detach after a successful attach, so
+ * tr->cur_image (captured above as old_image) is always
+ * non-NULL here; the NULL check only mirrors the one in
+ * bpf_trampoline_multi_attach_free()/_rollback()'s shared
+ * pattern and guards against tr->multi_attach being reused
+ * without a prior _init() call.
+ *
+ * This relies on update_ftrace_direct_del/mod being atomic:
+ * on failure, NO IPs in the hash are modified in ftrace (all
+ * validation/allocation happens before any ftrace record is
+ * touched). If this assumption is broken in the future (i.e.,
+ * partial success becomes possible), this rollback logic would
+ * need to be revisited.
+ *
+ * cur_image == NULL indicates the unreg path (total == 0);
+ * cur_image != NULL indicates the modify path (total > 0).
+ *
+ * Rollback alone only prevents freeing the trampoline image
+ * while ftrace may still branch into it; it does not keep
+ * the underlying bpf_prog alive, and the caller tears down
+ * link->prog once this function returns. So pin @prog (whose
+ * call is baked into old_image's machine code) on old_image
+ * before restoring it as cur_image: the pin is released once
+ * old_image is eventually retired for real by a later,
+ * successful update on this trampoline (see
+ * bpf_trampoline_multi_attach_free() and
+ * bpf_tramp_image_free()), or safely leaked alongside the
+ * image if the trampoline is torn down first instead (see
+ * bpf_trampoline_put()).
+ */
+ if (tr->multi_attach.old_image &&
+ tr->multi_attach.old_image != tr->cur_image &&
+ ((err_unreg && !tr->cur_image) ||
+ (err_mod && tr->cur_image))) {
+ WARN_ON_ONCE(tr->multi_attach.old_image->pinned_prog);
+ bpf_prog_inc(prog);
+ tr->multi_attach.old_image->pinned_prog = prog;
+ bpf_trampoline_multi_attach_rollback(tr);
+ } else {
+ bpf_trampoline_multi_attach_free(tr);
+ }
+ }
trampoline_unlock_all();
@@ -1749,7 +1842,6 @@ int bpf_trampoline_multi_detach(struct bpf_prog *prog,
struct bpf_tracing_multi_
bpf_trampoline_put(mnode->trampoline);
clear_tracing_multi_data(data);
- return 0;
}
#undef for_each_mnode_cnt
diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index 891897f8a1b3..29260951aa87 100644
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c
@@ -3687,7 +3687,7 @@ static void bpf_tracing_multi_link_release(struct
bpf_link *link)
struct bpf_tracing_multi_link *tr_link =
container_of(link, struct bpf_tracing_multi_link, link);
- WARN_ON_ONCE(bpf_trampoline_multi_detach(link->prog, tr_link));
+ bpf_trampoline_multi_detach(link->prog, tr_link);
}
static void bpf_tracing_multi_link_dealloc(struct bpf_link *link)
--
2.53.0
