From: Dave Marchevsky davemarchevsky@fb.com
commit 06ab134ce8ecfa5a69e850f88f81c8a4c3fa91df upstream.
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 Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- kernel/bpf/stackmap.c | 12 ++++++++++-- 1 file changed, 10 insertions(+), 2 deletions(-)
--- a/kernel/bpf/stackmap.c +++ b/kernel/bpf/stackmap.c @@ -662,9 +662,17 @@ const struct bpf_func_proto bpf_get_stac 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)