On Fri, May 13, 2022 at 12:08 PM Tadeusz Struk tadeusz.struk@linaro.org wrote:
Syzbot found a Use After Free bug in compute_effective_progs(). The reproducer creates a number of BPF links, and causes a fault injected alloc to fail, while calling bpf_link_detach on them. Link detach triggers the link to be freed by bpf_link_free(), which calls __cgroup_bpf_detach() and update_effective_progs(). If the memory allocation in this function fails, the function restores the pointer to the bpf_cgroup_link on the cgroup list, but the memory gets freed just after it returns. After this, every subsequent call to update_effective_progs() causes this already deallocated pointer to be dereferenced in prog_list_length(), and triggers KASAN UAF error.
To fix this issue don't preserve the pointer to the prog or link in the list, but remove it and replace it with a dummy prog without shrinking the table. The subsequent call to __cgroup_bpf_detach() or __cgroup_bpf_detach() will correct it.
Cc: "Alexei Starovoitov" ast@kernel.org Cc: "Daniel Borkmann" daniel@iogearbox.net Cc: "Andrii Nakryiko" andrii@kernel.org Cc: "Martin KaFai Lau" kafai@fb.com Cc: "Song Liu" songliubraving@fb.com Cc: "Yonghong Song" yhs@fb.com Cc: "John Fastabend" john.fastabend@gmail.com Cc: "KP Singh" kpsingh@kernel.org Cc: netdev@vger.kernel.org Cc: bpf@vger.kernel.org Cc: stable@vger.kernel.org Cc: linux-kernel@vger.kernel.org
Link: https://syzkaller.appspot.com/bug?id=8ebf179a95c2a2670f7cf1ba62429ec044369db... Fixes: af6eea57437a ("bpf: Implement bpf_link-based cgroup BPF program attachment") Reported-by: syzbot+f264bffdfbd5614f3bb2@syzkaller.appspotmail.com Signed-off-by: Tadeusz Struk tadeusz.struk@linaro.org
v2: Add a fall back path that removes a prog from the effective progs table in case detach fails to allocate memory in compute_effective_progs().
v3: Implement the fallback in a separate function purge_effective_progs
kernel/bpf/cgroup.c | 64 +++++++++++++++++++++++++++++++++++++++------ 1 file changed, 56 insertions(+), 8 deletions(-)
diff --git a/kernel/bpf/cgroup.c b/kernel/bpf/cgroup.c index 128028efda64..9d3af4d6c055 100644 --- a/kernel/bpf/cgroup.c +++ b/kernel/bpf/cgroup.c @@ -681,6 +681,57 @@ static struct bpf_prog_list *find_detach_entry(struct list_head *progs, return ERR_PTR(-ENOENT); }
+/**
- purge_effective_progs() - After compute_effective_progs fails to alloc new
cgrp->bpf.inactive table we can recover by
recomputing the array in place.
- @cgrp: The cgroup which descendants to traverse
- @link: A link to detach
- @atype: Type of detach operation
- */
+static void purge_effective_progs(struct cgroup *cgrp, struct bpf_prog *prog,
enum cgroup_bpf_attach_type atype)+{
struct cgroup_subsys_state *css;struct bpf_prog_array_item *item;struct bpf_prog *tmp;struct bpf_prog_array *array;int index = 0, index_purge = -1;if (!prog)return;/* recompute effective prog array in place */css_for_each_descendant_pre(css, &cgrp->self) {struct cgroup *desc = container_of(css, struct cgroup, self);array = desc->bpf.effective[atype];
../kernel/bpf/cgroup.c:748:23: warning: incorrect type in assignment (different address spaces) ../kernel/bpf/cgroup.c:748:23: expected struct bpf_prog_array *array ../kernel/bpf/cgroup.c:748:23: got struct bpf_prog_array [noderef] __rcu *
you need rcu_dereference here? but also see suggestions below to avoid iterating effective directly (it's ambiguous to search by prog only)
item = &array->items[0];/* Find the index of the prog to purge */while ((tmp = READ_ONCE(item->prog))) {if (tmp == prog) {
I think comparing just prog isn't always correct, as the same program can be in effective array multiple times if attached through bpf_link.
Looking at replace_effective_prog() I think we can do a very similar (and tested) approach:
1. restore original pl state in __cgroup_bpf_detach (so we can find it by comparing pl->prog == prog && pl->link == link) 2. use replace_effective_prog's approach to find position of pl in effective array (using this nested for loop over cgroup parents and list_for_each_entry inside) 3. then instead of replacing one prog with another do bpf_prog_array_delete_safe_at ?
I'd feel more comfortable using the same tested overall approach of replace_effective_prog.
index_purge = index;break;}item++;index++;}/* Check if we found what's needed for removing the prog */if (index_purge == -1 || index_purge == index - 1)continue;
the search shouldn't fail, should it?
/* Remove the program from the array */WARN_ONCE(bpf_prog_array_delete_safe_at(array, index_purge),"Failed to purge a prog from array at index %d", index_purge);index = 0;index_purge = -1;}+}
/**
- __cgroup_bpf_detach() - Detach the program or link from a cgroup, and
propagate the change to descendants@@ -723,8 +774,11 @@ static int __cgroup_bpf_detach(struct cgroup *cgrp, struct bpf_prog *prog, pl->link = NULL;
err = update_effective_progs(cgrp, atype);
if (err)goto cleanup;
if (err) {struct bpf_prog *prog_purge = prog ? prog : link->link.prog;
so here we shouldn't forget link, instead pass both link and prog (one of them will have to be NULL) into purge_effective_progs
purge_effective_progs(cgrp, prog_purge, atype);} /* now can actually delete it from this cgroup list */ list_del(&pl->node);@@ -736,12 +790,6 @@ static int __cgroup_bpf_detach(struct cgroup *cgrp, struct bpf_prog *prog, bpf_prog_put(old_prog); static_branch_dec(&cgroup_bpf_enabled_key[atype]); return 0;
-cleanup:
/* restore back prog or link */pl->prog = old_prog;pl->link = link;return err;}
static int cgroup_bpf_detach(struct cgroup *cgrp, struct bpf_prog *prog,
2.36.1