| Message ID | 4ff12d0c19de63e7172d25922adfb83ae7c8691f.1604620776.git.dxu@dxuuu.xyz (mailing list archive) |
|---|---|
| State | Changes Requested |
| Commit | baca7f1c1c1e91c4ada934e36a900666d13a6d30 |
| Delegated to: | BPF |
| Headers | show |
| Series | [bpf,v4,1/2] lib/strncpy_from_user.c: Don't overcopy bytes after NUL terminator | expand |
> On Nov 5, 2020, at 4:06 PM, Daniel Xu <dxu@dxuuu.xyz> wrote: > > 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@dxuuu.xyz> Acked-by: Song Liu <songliubraving@fb.com> [...]
Hello: This series was applied to bpf/bpf.git (refs/heads/master): On Thu, 5 Nov 2020 16:06:34 -0800 you wrote: > 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. > > [...] Here is the summary with links: - [bpf,v4,1/2] lib/strncpy_from_user.c: Don't overcopy bytes after NUL terminator https://git.kernel.org/bpf/bpf/c/baca7f1c1c1e - [bpf,v4,2/2] selftest/bpf: Test bpf_probe_read_user_str() strips trailing bytes after NUL https://git.kernel.org/bpf/bpf/c/d18b184127de You are awesome, thank you! -- Deet-doot-dot, I am a bot. https://korg.docs.kernel.org/patchwork/pwbot.html
diff --git a/lib/strncpy_from_user.c b/lib/strncpy_from_user.c index e6d5fcc2cdf3..82a67dde136b 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); } + + *(unsigned long *)(dst+res) = c; + res += sizeof(unsigned long); max -= sizeof(unsigned long); }
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@dxuuu.xyz> --- lib/strncpy_from_user.c | 9 +++++++-- 1 file changed, 7 insertions(+), 2 deletions(-)