[RFC PATCH 03/10] bpf: Fail sleepable user copy helpers when current->mm is NULL

[James Houghton <jthoughton@google.com>]

Sleepable BPF programs can run on behalf of a kernel thread, or of a
task that has already gone through exit_mm(). For example, bpf_wq
callbacks always run from a kworker, bpf_task_work callbacks queued
late enough run from exit_task_work(), and lsm.s or fentry.s programs
may be attached to functions called during exit_files(),
exit_task_work() or from kernel threads (e.g. security_file_release()
from a kworker running delayed_fput()).

In such contexts, bpf_copy_from_user(), bpf_copy_from_user_str() and
bpf_copy_from_user{,_str}_dynptr() (when reading from the current task)
call copy_from_user() or strncpy_from_user() without an mm. That access
does not reliably fail: the user half of the address space belongs to
whatever mm happens to be lazily loaded, if any, so the program may read
another task's memory.

As there is no user memory to read from in this case, fail with -EFAULT
without attempting the access.

Fixes: 07be4c4a3e7a ("bpf: Add bpf_copy_from_user() helper.")
Fixes: 65ab5ac4df01 ("bpf: Add bpf_copy_from_user_str kfunc")
Fixes: a498ee7576de ("bpf: Implement dynptr copy kfuncs")
Signed-off-by: James Houghton <jthoughton@google.com>
---
Tested with a small bpf_wq reproducer: a SEC("syscall") program arms a
bpf_wq whose callback (which runs in a kworker) calls
bpf_copy_from_user() on an address in the loader. With the uaccess check
from later in this series but without this patch, that triggers the new
WARN from bpf_copy_from_user(); with this patch, the helper returns
-EFAULT without warning. I can turn this into a selftest under
tools/testing/selftests/bpf if that's useful.
---
 kernel/bpf/helpers.c     | 11 +++++++++--
 kernel/trace/bpf_trace.c |  4 ++++
 2 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index 712dca5a2c5b..c4aa8bcaf2bd 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -659,7 +659,11 @@ const struct bpf_func_proto bpf_event_output_data_proto =  {
 BPF_CALL_3(bpf_copy_from_user, void *, dst, u32, size,
 	   const void __user *, user_ptr)
 {
-	int ret = copy_from_user(dst, user_ptr, size);
+	int ret = -EFAULT;
+
+	/* Sleepable programs may run without an mm, e.g. on kernel threads. */
+	if (likely(current->mm))
+		ret = copy_from_user(dst, user_ptr, size);
 
 	if (unlikely(ret)) {
 		memset(dst, 0, size);
@@ -3642,7 +3646,10 @@ __bpf_kfunc int bpf_copy_from_user_str(void *dst, u32 dst__sz, const void __user
 	if (unlikely(!dst__sz))
 		return 0;
 
-	ret = strncpy_from_user(dst, unsafe_ptr__ign, dst__sz - 1);
+	if (unlikely(!current->mm))
+		ret = -EFAULT;
+	else
+		ret = strncpy_from_user(dst, unsafe_ptr__ign, dst__sz - 1);
 	if (ret < 0) {
 		if (flags & BPF_F_PAD_ZEROS)
 			memset((char *)dst, 0, dst__sz);
diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index 88c3b5b513ab..793fb3dce5f1 100644
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c
@@ -3556,6 +3556,8 @@ static __always_inline int copy_user_data_sleepable(void *dst, const void *unsaf
 	int ret;
 
 	if (!tsk) { /* Read from the current task */
+		if (unlikely(!current->mm))
+			return -EFAULT;
 		ret = copy_from_user(dst, (const void __user *)unsafe_src, size);
 		if (ret)
 			return -EFAULT;
@@ -3590,6 +3592,8 @@ static __always_inline int copy_user_str_sleepable(void *dst, const void *unsafe
 
 	if (tsk) {
 		ret = copy_remote_vm_str(tsk, (unsigned long)unsafe_src, dst, size, 0);
+	} else if (unlikely(!current->mm)) {
+		ret = -EFAULT;
 	} else {
 		ret = strncpy_from_user(dst, (const void __user *)unsafe_src, size - 1);
 		/* strncpy_from_user does not guarantee NUL termination */
-- 
2.56.0.307.g5fe7bd680c-goog