[PATCH 4/4] block/vvfat: Avoid out of bounds write in create_long_filename()

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

The direntry_t::name holds 11 bytes:

  typedef struct direntry_t {
      uint8_t name[8 + 3];
      ...

However create_long_filename() writes up to 31 bytes into it:

 421     for(i=0;i<26*number_of_entries;i++) {
 422         int offset=(i%26);
 423         if(offset<10) offset=1+offset;
 424         else if(offset<22) offset=14+offset-10;
 425         else offset=28+offset-22;
 426         entry=array_get(&(s->directory),s->directory.next-1-(i/26));
 427         if (i >= 2 * length + 2) {
 428             entry->name[offset] = 0xff;
 429         } else if (i % 2 == 0) {
 430             entry->name[offset] = longname[i / 2] & 0xff;
 431         } else {
 432             entry->name[offset] = longname[i / 2] >> 8;
 433         }
 434     }

For example, if i=25, offset=28+25-22=31

Then in lines 428, 430 and 432 the entry->name[] array is written beside
its 11 bytes, as reported by Clang sanitizer:

  block/vvfat.c:430:13: runtime error: index 14 out of bounds for type 'uint8_t [11]'
  SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior block/vvfat.c:430:13 in
  block/vvfat.c:432:13: runtime error: index 15 out of bounds for type 'uint8_t [11]'
  SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior block/vvfat.c:432:13 in
  block/vvfat.c:428:13: runtime error: index 18 out of bounds for type 'uint8_t [11]'
  SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior block/vvfat.c:428:13 in

As I have no idea about what this code does, simply skip the writes if
out of range, since it is not worst than what we have currently (and
my tests using vvfat work identically).

Fixes: de167e416fa ("Virtual VFAT support (Johannes Schindelin)")
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
---
 block/vvfat.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/block/vvfat.c b/block/vvfat.c
index c193a816646..c7162e77d68 100644
--- a/block/vvfat.c
+++ b/block/vvfat.c
@@ -423,6 +423,9 @@ static direntry_t *create_long_filename(BDRVVVFATState *s, const char *filename)
         if(offset<10) offset=1+offset;
         else if(offset<22) offset=14+offset-10;
         else offset=28+offset-22;
+        if (offset >= ARRAY_SIZE(entry->name)) {
+            continue;
+        }
         entry=array_get(&(s->directory),s->directory.next-1-(i/26));
         if (i >= 2 * length + 2) {
             entry->name[offset] = 0xff;
-- 
2.26.3