[PATCH bpf-next] bpf: Remove bpf_lsm_file_mprotect from sleepable list.

[PATCH bpf-next] bpf: Remove bpf_lsm_file_mprotect from sleepable list.

[Alexei Starovoitov]

From: Alexei Starovoitov <ast@kernel.org>

Technically the bpf programs can sleep while attached to bpf_lsm_file_mprotect,
but such programs need to access user memory. So they're in might_fault()
category. Which means they cannot be called from file_mprotect lsm hook that
takes write lock on mm->mmap_lock.
Adjust the test accordingly.

Also add might_fault() to __bpf_prog_enter_sleepable() to catch such deadlocks early.

Reported-by: Yonghong Song <yhs@fb.com>
Fixes: 1e6c62a88215 ("bpf: Introduce sleepable BPF programs")
Fixes: e68a144547fc ("selftests/bpf: Add sleepable tests")
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
---
 kernel/bpf/trampoline.c                 |  1 +
 kernel/bpf/verifier.c                   |  1 -
 tools/testing/selftests/bpf/progs/lsm.c | 34 ++++++++++++-------------
 3 files changed, 18 insertions(+), 18 deletions(-)

diff --git a/kernel/bpf/trampoline.c b/kernel/bpf/trampoline.c
index c2b76545153c..7dd523a7e32d 100644
--- a/kernel/bpf/trampoline.c
+++ b/kernel/bpf/trampoline.c
@@ -409,6 +409,7 @@ void notrace __bpf_prog_exit(struct bpf_prog *prog, u64 start)
 void notrace __bpf_prog_enter_sleepable(void)
 {
 	rcu_read_lock_trace();
+	might_fault();
 }
 
 void notrace __bpf_prog_exit_sleepable(void)
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index b4c22b5ce5a2..b4e9c56b8b32 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -11006,7 +11006,6 @@ static int check_attach_modify_return(struct bpf_prog *prog, unsigned long addr)
 /* non exhaustive list of sleepable bpf_lsm_*() functions */
 BTF_SET_START(btf_sleepable_lsm_hooks)
 #ifdef CONFIG_BPF_LSM
-BTF_ID(func, bpf_lsm_file_mprotect)
 BTF_ID(func, bpf_lsm_bprm_committed_creds)
 #else
 BTF_ID_UNUSED
diff --git a/tools/testing/selftests/bpf/progs/lsm.c b/tools/testing/selftests/bpf/progs/lsm.c
index 49fa6ca99755..ff4d343b94b5 100644
--- a/tools/testing/selftests/bpf/progs/lsm.c
+++ b/tools/testing/selftests/bpf/progs/lsm.c
@@ -36,14 +36,10 @@ int monitored_pid = 0;
 int mprotect_count = 0;
 int bprm_count = 0;
 
-SEC("lsm.s/file_mprotect")
+SEC("lsm/file_mprotect")
 int BPF_PROG(test_int_hook, struct vm_area_struct *vma,
 	     unsigned long reqprot, unsigned long prot, int ret)
 {
-	char args[64];
-	__u32 key = 0;
-	__u64 *value;
-
 	if (ret != 0)
 		return ret;
 
@@ -53,18 +49,6 @@ int BPF_PROG(test_int_hook, struct vm_area_struct *vma,
 	is_stack = (vma->vm_start <= vma->vm_mm->start_stack &&
 		    vma->vm_end >= vma->vm_mm->start_stack);
 
-	bpf_copy_from_user(args, sizeof(args), (void *)vma->vm_mm->arg_start);
-
-	value = bpf_map_lookup_elem(&array, &key);
-	if (value)
-		*value = 0;
-	value = bpf_map_lookup_elem(&hash, &key);
-	if (value)
-		*value = 0;
-	value = bpf_map_lookup_elem(&lru_hash, &key);
-	if (value)
-		*value = 0;
-
 	if (is_stack && monitored_pid == pid) {
 		mprotect_count++;
 		ret = -EPERM;
@@ -77,10 +61,26 @@ SEC("lsm.s/bprm_committed_creds")
 int BPF_PROG(test_void_hook, struct linux_binprm *bprm)
 {
 	__u32 pid = bpf_get_current_pid_tgid() >> 32;
+	char args[64];
+	__u32 key = 0;
+	__u64 *value;
 
 	if (monitored_pid == pid)
 		bprm_count++;
 
+	bpf_copy_from_user(args, sizeof(args), (void *)bprm->vma->vm_mm->arg_start);
+	bpf_copy_from_user(args, sizeof(args), (void *)bprm->mm->arg_start);
+
+	value = bpf_map_lookup_elem(&array, &key);
+	if (value)
+		*value = 0;
+	value = bpf_map_lookup_elem(&hash, &key);
+	if (value)
+		*value = 0;
+	value = bpf_map_lookup_elem(&lru_hash, &key);
+	if (value)
+		*value = 0;
+
 	return 0;
 }
 SEC("lsm/task_free") /* lsm/ is ok, lsm.s/ fails */
-- 
2.23.0

Re: [PATCH bpf-next] bpf: Remove bpf_lsm_file_mprotect from sleepable list.

[Daniel Borkmann]

On 8/31/20 10:16 PM, Alexei Starovoitov wrote:
> From: Alexei Starovoitov <ast@kernel.org>
> 
> Technically the bpf programs can sleep while attached to bpf_lsm_file_mprotect,
> but such programs need to access user memory. So they're in might_fault()
> category. Which means they cannot be called from file_mprotect lsm hook that
> takes write lock on mm->mmap_lock.
> Adjust the test accordingly.
> 
> Also add might_fault() to __bpf_prog_enter_sleepable() to catch such deadlocks early.
> 
> Reported-by: Yonghong Song <yhs@fb.com>
> Fixes: 1e6c62a88215 ("bpf: Introduce sleepable BPF programs")
> Fixes: e68a144547fc ("selftests/bpf: Add sleepable tests")
> Signed-off-by: Alexei Starovoitov <ast@kernel.org>
> ---
>   kernel/bpf/trampoline.c                 |  1 +
>   kernel/bpf/verifier.c                   |  1 -
>   tools/testing/selftests/bpf/progs/lsm.c | 34 ++++++++++++-------------
>   3 files changed, 18 insertions(+), 18 deletions(-)
> 
> diff --git a/kernel/bpf/trampoline.c b/kernel/bpf/trampoline.c
> index c2b76545153c..7dd523a7e32d 100644
> --- a/kernel/bpf/trampoline.c
> +++ b/kernel/bpf/trampoline.c
> @@ -409,6 +409,7 @@ void notrace __bpf_prog_exit(struct bpf_prog *prog, u64 start)
>   void notrace __bpf_prog_enter_sleepable(void)
>   {
>   	rcu_read_lock_trace();
> +	might_fault();

Makes sense, was wondering about a __might_sleep() but that will cover it internally
too. Applied, thanks!

>   }
>   
>   void notrace __bpf_prog_exit_sleepable(void)