Re: [PATCH 1/2] vvfat: Check that updated filenames are valid

[Kevin Wolf <kwolf@redhat.com>]

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 <nhuck15@gmail.com>
> > Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> > ---
> >   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