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

[Eric Blake]

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/

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"

> +        {
> +            return false;
> +        }
> +    }
> +    return true;
> +}
> +
>   static uint8_t to_valid_short_char(gunichar c)
>   {
>       c = g_unichar_toupper(c);
> @@ -2098,6 +2117,10 @@ DLOG(fprintf(stderr, "check direntry %d:\n", i); 
> print_direntry(direntries + i))
>               }
>               lfn.checksum = 0x100; /* cannot use long name twice */
>   
> +            if (!valid_filename(lfn.name)) {
> +                fprintf(stderr, "Invalid file name\n");

Wow, the fact that we are still using fprintf is annoying, but pre-existing.

> +                goto fail;
> +            }
>               if (path_len + 1 + lfn.len >= PATH_MAX) {
>                   fprintf(stderr, "Name too long: %s/%s\n", path, lfn.name);
>                   goto fail;

At any rate, the idea makes sense. If you don't like my strcspn() idea, 
then:
Reviewed-by: Eric Blake <eblake@redhat.com>

--
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org