[PATCH bpf v6 1/2] lib/strncpy_from_user.c: Don't overcopy bytes after NUL terminator

[Daniel Xu]

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.

Fixes: 6ae08ae3dea2 ("bpf: Add probe_read_{user, kernel} and probe_read_{user, kernel}_str helpers")
Signed-off-by: Daniel Xu <dxu@xxxxxxxxx>
---
Based on on-list discussion and some off-list discussion with Alexei,
I'd like to propose the v4-style patch without the `(*out & ~mask)`
bit.

We can't really zero out the rest of the buffer due to ABI issues.
The bpf docs state for bpf_probe_read_user_str():

> In case the string length is smaller than *size*, the target is not
> padded with further NUL bytes.

 lib/strncpy_from_user.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/lib/strncpy_from_user.c b/lib/strncpy_from_user.c
index e6d5fcc2cdf3..de084f04e50d 100644
--- a/lib/strncpy_from_user.c
+++ b/lib/strncpy_from_user.c
@@ -35,17 +35,21 @@ 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;
 
 		/* 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);
+			*(unsigned long *)(dst+res) = c & mask;
 			return res + find_zero(data);
 		}
+
+		*(unsigned long *)(dst+res) = c;
+
 		res += sizeof(unsigned long);
 		max -= sizeof(unsigned long);
 	}
-- 
2.29.2