[PATCH bpf-next] lib/strncpy_from_user.c: Don't overcopy bytes after NUL terminator

[Daniel Xu <dxu@dxuuu.xyz>]

do_strncpy_from_user() may copy some extra bytes after the NUL
terminator into the destination buffer. This usually does not matter for
normal string operations. However, when BPF programs key BPF maps with
strings, this matters a lot.

A BPF program may read strings from user memory by calling the
bpf_probe_read_user_str() helper which eventually calls
do_strncpy_from_user(). The program can then key a map with the
resulting string. BPF map keys are fixed-width and string-agnostic,
meaning that map keys are treated as a set of bytes.

The issue is when do_strncpy_from_user() overcopies bytes after the NUL
terminator, it can result in seemingly identical strings occupying
multiple slots in a BPF map. This behavior is subtle and totally
unexpected by the user.

This commit uses the proper word-at-a-time APIs to avoid overcopying.

Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
---
 lib/strncpy_from_user.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/lib/strncpy_from_user.c b/lib/strncpy_from_user.c
index e6d5fcc2cdf3..d084189eb05c 100644
--- a/lib/strncpy_from_user.c
+++ b/lib/strncpy_from_user.c
@@ -35,17 +35,22 @@ static inline long do_strncpy_from_user(char *dst, const char __user *src,
 		goto byte_at_a_time;
 
 	while (max >= sizeof(unsigned long)) {
-		unsigned long c, data;
+		unsigned long c, data, mask, *out;
 
 		/* Fall back to byte-at-a-time if we get a page fault */
 		unsafe_get_user(c, (unsigned long __user *)(src+res), byte_at_a_time);
 
-		*(unsigned long *)(dst+res) = c;
 		if (has_zero(c, &data, &constants)) {
 			data = prep_zero_mask(c, data, &constants);
 			data = create_zero_mask(data);
+			mask = zero_bytemask(data);
+			out = (unsigned long *)(dst+res);
+			*out = (*out & ~mask) | (c & mask);
 			return res + find_zero(data);
+		} else  {
+			*(unsigned long *)(dst+res) = c;
 		}
+
 		res += sizeof(unsigned long);
 		max -= sizeof(unsigned long);
 	}
-- 
2.28.0