[PATCH] fs/proc: fix the write length returned by proc_sys_call_handler()

[Cheng Yongkang]

proc_sys_call_handler() reads the write length from iov_iter_count() and
copies that many bytes from the iterator into kbuf.  A cgroup/sysctl BPF
program can replace the new value through bpf_sysctl_set_new_value() and
update count to the length of the replacement value.  The handler then
returns that adjusted count as the number of bytes written, even though
the iterator originally held a different number of bytes.

If the replacement value is longer than the original input, ->write_iter()
reports more bytes than the iterator held.  splice(2) passes that value to
iter_file_splice_write(), which subtracts it from the pipe length and
walks past the pipe head:

  Splice Exceeded! ret=7 tot=1 left=0
  Oops: general protection fault
  RIP: 0010:iter_file_splice_write+0xb6f/0x1170

The write path consumes at most the original iterator length in
copy_from_iter_full() before the BPF hook runs.  Return the smaller of the
adjusted and original counts on the write path, so an expanded BPF value
cannot exceed the iterator while a handler that reports a shorter parsed
length still produces a short write.  The read path keeps using the
adjusted count for copy_to_iter().

Fixes: 4e63acdff864 ("bpf: Introduce bpf_sysctl_{get,set}_new_value helpers")
Cc: stable@xxxxxxxxxxxxxxx
Assisted-by: LLM
Signed-off-by: Cheng Yongkang <teel4res@xxxxxxxxx>
---
 fs/proc/proc_sysctl.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c
index 6c0859c..eeebf81 100644
--- a/fs/proc/proc_sysctl.c
+++ b/fs/proc/proc_sysctl.c
@@ -557,6 +557,8 @@ static ssize_t proc_sys_call_handler(struct kiocb *iocb, struct iov_iter *iter,
 	struct ctl_table_header *head = grab_header(inode);
 	const struct ctl_table *table = PROC_I(inode)->sysctl_entry;
 	size_t count = iov_iter_count(iter);
+	/* ->write_iter() must never report more than the iterator held */
+	size_t orig_count = count;
 	char *kbuf;
 	ssize_t error;
 
@@ -607,7 +609,8 @@ static ssize_t proc_sys_call_handler(struct kiocb *iocb, struct iov_iter *iter,
 			goto out_free_buf;
 	}
 
-	error = count;
+	/* the write path consumed at most the original iterator length */
+	error = write ? min(count, orig_count) : count;
 out_free_buf:
 	kvfree(kbuf);
 out:
-- 
2.34.1