[PATCH bpf-next v4 3/3] bpf: use copy_struct_from_user() in bpf() syscall

[Christian Brauner]

In v5.4-rc2 we added a new helper (cf. [1]) copy_struct_from_user().
This helper is intended for all codepaths that copy structs from
userspace that are versioned by size. The bpf() syscall does exactly
what copy_struct_from_user() is doing.
Note that copy_struct_from_user() is calling min() already. So
technically, the min_t() call could go. But the size is used further
below so leave it.

[1]: f5a1a536fa14 ("lib: introduce copy_struct_from_user() helper")
Cc: Alexei Starovoitov <ast@xxxxxxxxxx>
Cc: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
Cc: Aleksa Sarai <cyphar@xxxxxxxxxx>
Cc: bpf@xxxxxxxxxxxxxxx
Signed-off-by: Christian Brauner <christian.brauner@xxxxxxxxxx>
---
/* v1 */
Link: https://lore.kernel.org/r/20191009160907.10981-4-christian.brauner@xxxxxxxxxx

/* v2 */
Link: https://lore.kernel.org/r/20191016004138.24845-4-christian.brauner@xxxxxxxxxx
- Alexei Starovoitov <ast@xxxxxxxxxx>:
  - remove unneeded initialization

/* v3 */
Link: https://lore.kernel.org/r/20191016034432.4418-4-christian.brauner@xxxxxxxxxx
unchanged

/* v4 */
- Alexei Starovoitov <ast@xxxxxxxxxx>:
  - move misplaced min after copy_struct_from_user()
---
 kernel/bpf/syscall.c | 9 +++------
 1 file changed, 3 insertions(+), 6 deletions(-)

diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index d554ca7671b6..47bf4a81002d 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -2812,21 +2812,18 @@ static int bpf_task_fd_query(const union bpf_attr *attr,
 
 SYSCALL_DEFINE3(bpf, int, cmd, union bpf_attr __user *, uattr, unsigned int, size)
 {
-	union bpf_attr attr = {};
+	union bpf_attr attr;
 	int err;
 
 	if (sysctl_unprivileged_bpf_disabled && !capable(CAP_SYS_ADMIN))
 		return -EPERM;
 
-	err = bpf_check_uarg_tail_zero(uattr, sizeof(attr), size);
+	/* copy attributes from user space, may be less than sizeof(bpf_attr) */
+	err = copy_struct_from_user(&attr, sizeof(attr), uattr, size);
 	if (err)
 		return err;
 	size = min_t(u32, size, sizeof(attr));
 
-	/* copy attributes from user space, may be less than sizeof(bpf_attr) */
-	if (copy_from_user(&attr, uattr, size) != 0)
-		return -EFAULT;
-
 	err = security_bpf(cmd, &attr, size);
 	if (err < 0)
 		return err;
-- 
2.23.0