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Cc:     "bpf" <bpf@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "ast@kernel.org" <ast@kernel.org>,
        "daniel@iogearbox.net" <daniel@iogearbox.net>,
        "Kernel Team" <Kernel-team@fb.com>
Subject: Re: [PATCH bpf v2 1/2] lib/strncpy_from_user.c: Don't overcopy
 bytes after NUL terminator
From:   "Daniel Xu" <dxu@dxuuu.xyz>
To:     "Song Liu" <songliubraving@fb.com>
Date:   Thu, 05 Nov 2020 11:28:11 -0800
Message-Id: <C6VKTIUUWPD0.2INCG5OQMYVNJ@maharaja>
In-Reply-To: <CE6BCF1F-2112-40DC-87C8-91FA2D6C86FC@fb.com>
Precedence: bulk

On Thu Nov 5, 2020 at 10:16 AM PST, Song Liu wrote:
>
>
> > On Nov 4, 2020, at 6:25 PM, Daniel Xu <dxu@dxuuu.xyz> wrote:
> >=20
> > do_strncpy_from_user() may copy some extra bytes after the NUL
>
> We have multiple use of "NUL" here, should be "NULL"?
>
> > terminator into the destination buffer. This usually does not matter fo=
r
> > normal string operations. However, when BPF programs key BPF maps with
> > strings, this matters a lot.
> >=20
> > 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.
> >=20
> > 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.
> >=20
> > This commit uses the proper word-at-a-time APIs to avoid overcopying.
> >=20
> > 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(-)
> >=20
> > 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;
> >=20
> > 	while (max >=3D sizeof(unsigned long)) {
> > -		unsigned long c, data;
> > +		unsigned long c, data, mask, *out;
> >=20
> > 		/* 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)=
;
> >=20
> > -		*(unsigned long *)(dst+res) =3D c;
> > 		if (has_zero(c, &data, &constants)) {
> > 			data =3D prep_zero_mask(c, data, &constants);
> > 			data =3D create_zero_mask(data);
> > +			mask =3D zero_bytemask(data);
> > +			out =3D (unsigned long *)(dst+res);
> > +			*out =3D (*out & ~mask) | (c & mask);
> > 			return res + find_zero(data);
> > +		} else  {
>
> This else clause is not needed, as we return in the if clause.

Thanks, will change in v3.

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