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Wed, 24 Jun 2020 12:36:36 +0000 (UTC) Date: Wed, 24 Jun 2020 14:36:35 +0200 From: Kevin Wolf To: Eric Blake Subject: Re: [PATCH 1/2] vvfat: Check that updated filenames are valid Message-ID: <20200624123635.GB9253@linux.fritz.box> References: <20200623175534.38286-1-kwolf@redhat.com> <20200623175534.38286-2-kwolf@redhat.com> <33941063-cfcc-002b-5fe8-d37050d8e532@redhat.com> MIME-Version: 1.0 In-Reply-To: <33941063-cfcc-002b-5fe8-d37050d8e532@redhat.com> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Received-SPF: pass client-ip=207.211.31.120; envelope-from=kwolf@redhat.com; helo=us-smtp-1.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/06/24 02:33:25 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] [fuzzy] X-Spam_score_int: -30 X-Spam_score: -3.1 X-Spam_bar: --- X-Spam_report: (-3.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-1, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H3=-0.01, RCVD_IN_MSPIKE_WL=-0.01, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=_AUTOLEARN X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: nhuck15@gmail.com, qemu-devel@nongnu.org, qemu-block@nongnu.org, ppandit@redhat.com Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" Am 23.06.2020 um 20:21 hat Eric Blake geschrieben: > On 6/23/20 12:55 PM, Kevin Wolf wrote: > > FAT allows only a restricted set of characters in file names, and for > > some of the illegal characters, it's actually important that we catch > > them: If filenames can contain '/', the guest can construct filenames > > containing "../" and escape from the assigned vvfat directory. The same > > problem could arise if ".." was ever accepted as a literal filename. > > > > Fix this by adding a check that all filenames are valid in > > check_directory_consistency(). > > > > Reported-by: Nathan Huckleberry > > Signed-off-by: Kevin Wolf > > --- > > block/vvfat.c | 23 +++++++++++++++++++++++ > > 1 file changed, 23 insertions(+) > > > > diff --git a/block/vvfat.c b/block/vvfat.c > > index c65a98e3ee..2fab371258 100644 > > --- a/block/vvfat.c > > +++ b/block/vvfat.c > > @@ -520,6 +520,25 @@ static void set_begin_of_direntry(direntry_t* direntry, uint32_t begin) > > direntry->begin_hi = cpu_to_le16((begin >> 16) & 0xffff); > > } > > +static bool valid_filename(const unsigned char *name) > > +{ > > + unsigned char c; > > + if (!strcmp((const char*)name, ".") || !strcmp((const char*)name, "..")) { > > + return false; > > + } > > + for (; (c = *name); name++) { > > + if (!((c >= '0' && c <= '9') || > > + (c >= 'A' && c <= 'Z') || > > + (c >= 'a' && c <= 'z') || > > + c > 127 || > > + strchr("$%'-_@~`!(){}^#&.+,;=[]", c) != 0)) > > s/0/NULL/ Ok, though this line is just copied from to_valid_short_char(). Maybe I can sneak in a (strictly speaking unrelated) change to that function to keep both consistent. > Hmm - would it be any more efficient to use a single comparison of strcspn() > vs. strlen(), where you merely spell out the bytes that are rejected? Out > of 256 byte values, NUL is implicitly rejected (since these are C strings), > the 128 high-bit bytes are all valid, and you have permitted 62 alnum and 23 > other characters; that leaves merely 42 byte values to explicitly list in a > reject string. Of course, writing the string literal containing those 42 > invalid bytes is itself a bit of an exercise in reading the ASCII table: > > "\x01\x02\x03\x04\x05\x06\x07" > "\x08\x09\x0a\x0b\x0c\x0d\x0e\x0f" > "\x10\x11\x12\x13\x14\x15\x16\x17" > "\x18\x19\x1a\x1b\x1c\x1d\x1e\x1f" > " \"*/:<>?\\|\x7f" I think this would be really hard to read. The above condition is a pretty straighforward implementation of what the spec says (even the order of characters is the same). Kevin