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authorDave Marchevsky <davemarchevsky@fb.com>2021-03-31 17:07:47 -0700
committerAlexei Starovoitov <ast@kernel.org>2021-04-01 13:58:07 -0700
commit06ab134ce8ecfa5a69e850f88f81c8a4c3fa91df (patch)
treea847d2640bbf5d0ee273b6e892684fd9c4d0964f /kernel
parent861de02e5f3f2a104eecc5af1d248cb7bf8c5f75 (diff)
bpf: Refcount task stack in bpf_get_task_stack
On x86 the struct pt_regs * grabbed by task_pt_regs() points to an offset of task->stack. The pt_regs are later dereferenced in __bpf_get_stack (e.g. by user_mode() check). This can cause a fault if the task in question exits while bpf_get_task_stack is executing, as warned by task_stack_page's comment: * When accessing the stack of a non-current task that might exit, use * try_get_task_stack() instead. task_stack_page will return a pointer * that could get freed out from under you. Taking the comment's advice and using try_get_task_stack() and put_task_stack() to hold task->stack refcount, or bail early if it's already 0. Incrementing stack_refcount will ensure the task's stack sticks around while we're using its data. I noticed this bug while testing a bpf task iter similar to bpf_iter_task_stack in selftests, except mine grabbed user stack, and getting intermittent crashes, which resulted in dumps like: BUG: unable to handle page fault for address: 0000000000003fe0 \#PF: supervisor read access in kernel mode \#PF: error_code(0x0000) - not-present page RIP: 0010:__bpf_get_stack+0xd0/0x230 <snip...> Call Trace: bpf_prog_0a2be35c092cb190_get_task_stacks+0x5d/0x3ec bpf_iter_run_prog+0x24/0x81 __task_seq_show+0x58/0x80 bpf_seq_read+0xf7/0x3d0 vfs_read+0x91/0x140 ksys_read+0x59/0xd0 do_syscall_64+0x48/0x120 entry_SYSCALL_64_after_hwframe+0x44/0xa9 Fixes: fa28dcb82a38 ("bpf: Introduce helper bpf_get_task_stack()") Signed-off-by: Dave Marchevsky <davemarchevsky@fb.com> Signed-off-by: Alexei Starovoitov <ast@kernel.org> Acked-by: Song Liu <songliubraving@fb.com> Link: https://lore.kernel.org/bpf/20210401000747.3648767-1-davemarchevsky@fb.com
Diffstat (limited to 'kernel')
-rw-r--r--kernel/bpf/stackmap.c12
1 files changed, 10 insertions, 2 deletions
diff --git a/kernel/bpf/stackmap.c b/kernel/bpf/stackmap.c
index be35bfb7fb13..6fbc2abe9c91 100644
--- a/kernel/bpf/stackmap.c
+++ b/kernel/bpf/stackmap.c
@@ -517,9 +517,17 @@ const struct bpf_func_proto bpf_get_stack_proto = {
BPF_CALL_4(bpf_get_task_stack, struct task_struct *, task, void *, buf,
u32, size, u64, flags)
{
- struct pt_regs *regs = task_pt_regs(task);
+ struct pt_regs *regs;
+ long res;
- return __bpf_get_stack(regs, task, NULL, buf, size, flags);
+ if (!try_get_task_stack(task))
+ return -EFAULT;
+
+ regs = task_pt_regs(task);
+ res = __bpf_get_stack(regs, task, NULL, buf, size, flags);
+ put_task_stack(task);
+
+ return res;
}
BTF_ID_LIST_SINGLE(bpf_get_task_stack_btf_ids, struct, task_struct)