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buffering: fix buffer overflows with bitmap loading

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In some circumstances it was possible for a bitmap to overflow its
buffer and overwrite the next handle. The easiest way to trigger it
is with a highly compressed JPEG that is decoded to a large bitmap.
Because the JPEG file size is used to determine how much to allocate
this would cause an obvious buffer overflow when the JPEG is smaller
than the decoded bitmap. Fix this by using the decoded bitmap size as
the allocation size. Some overhead must be added to deal with JPEGs,
but it will be freed once the image is loaded.

A less obvious possibility is the fact that add_handle() will allow
a handle to be added even if there's not enough space for the entire
allocation. This is generally beneficial because it allows the first
part of a file to be loaded while waiting for space to free up, but
for bitmaps it is not valid because the whole image is loaded at once.
Hence if there is not actually enough space in the buffer, the bitmap
load can again overflow the actual free space and overwrite the next
handle.

The buffering code supports an H_ALLOCALL flag for allocations that
need the free space available immediately, so use it for bitmaps to
avoid that bug.

load_image() had a sketchy-looking check for free space which stopped
me from triggering the bug with simple tests, but since guessing the
free space is obviously a bad idea when the caller *knows* how much
free space there really is, remove that guess and let the caller tell
load_image() the real deal.

Change-Id: If62a58759705d83c16ee5b50f26bcbccc3f6c01f
This commit is contained in:
Aidan MacDonald 2022-04-01 11:53:17 +01:00
parent 5aa0fc3b00
commit 32f1418c5a
1 changed files with 23 additions and 17 deletions
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40
apps/buffering.c
View file

@ -848,8 +848,9 @@ static bool fill_buffer(void)
Return value is the total size (struct + data). */
static int load_image(int fd, const char *path,
struct bufopen_bitmap_data *data,
size_t bufidx)
size_t bufidx, size_t max_size)
{
(void)path;
int rc;
struct bitmap *bmp = ringbuf_ptr(bufidx);
struct dim *dim = data->dim;
@ -863,25 +864,20 @@ static int load_image(int fd, const char *path,
#if (LCD_DEPTH > 1) || defined(HAVE_REMOTE_LCD) && (LCD_REMOTE_DEPTH > 1)
bmp->maskdata = NULL;
#endif
int free = (int)MIN(buffer_len - bytes_used(), buffer_len - bufidx)
- sizeof(struct bitmap);
const int format = FORMAT_NATIVE | FORMAT_DITHER |
FORMAT_RESIZE | FORMAT_KEEP_ASPECT;
#ifdef HAVE_JPEG
if (aa != NULL) {
lseek(fd, aa->pos, SEEK_SET);
rc = clip_jpeg_fd(fd, aa->size, bmp, free, FORMAT_NATIVE|FORMAT_DITHER|
FORMAT_RESIZE|FORMAT_KEEP_ASPECT, NULL);
rc = clip_jpeg_fd(fd, aa->size, bmp, (int)max_size, format, NULL);
}
else if (strcmp(path + strlen(path) - 4, ".bmp"))
rc = read_jpeg_fd(fd, bmp, free, FORMAT_NATIVE|FORMAT_DITHER|
FORMAT_RESIZE|FORMAT_KEEP_ASPECT, NULL);
rc = read_jpeg_fd(fd, bmp, (int)max_size, format, NULL);
else
#endif
rc = read_bmp_fd(fd, bmp, free, FORMAT_NATIVE|FORMAT_DITHER|
FORMAT_RESIZE|FORMAT_KEEP_ASPECT, NULL);
rc = read_bmp_fd(fd, bmp, (int)max_size, format, NULL);
return rc + (rc > 0 ? sizeof(struct bitmap) : 0);
(void)path;
}
#endif /* HAVE_ALBUMART */
@ -967,11 +963,18 @@ int bufopen(const char *file, off_t offset, enum data_type type,
size_t size = 0;
#ifdef HAVE_ALBUMART
if (type == TYPE_BITMAP) {
/* If albumart is embedded, the complete file is not buffered,
* but only the jpeg part; filesize() would be wrong */
/* Bitmaps are resized to the requested dimensions when loaded,
* so the file size should not be used as it may be too large
* or too small */
struct bufopen_bitmap_data *aa = user_data;
if (aa->embedded_albumart)
size = aa->embedded_albumart->size;
size = BM_SIZE(aa->dim->width, aa->dim->height, FORMAT_NATIVE, false);
size += sizeof(struct bitmap);
#ifdef HAVE_JPEG
/* JPEG loading requires extra memory
* TODO: don't add unncessary overhead for .bmp images! */
size += JPEG_DECODE_OVERHEAD;
#endif
}
#endif
@ -980,7 +983,10 @@ int bufopen(const char *file, off_t offset, enum data_type type,
unsigned int hflags = 0;
if (type == TYPE_PACKET_AUDIO || type == TYPE_CODEC)
hflags = H_CANWRAP;
hflags |= H_CANWRAP;
/* Bitmaps need their space allocated up front */
if (type == TYPE_BITMAP)
hflags |= H_ALLOCALL;
size_t adjusted_offset = offset;
if (adjusted_offset > size)
@ -1031,7 +1037,7 @@ int bufopen(const char *file, off_t offset, enum data_type type,
#ifdef HAVE_ALBUMART
if (type == TYPE_BITMAP) {
/* Bitmap file: we load the data instead of the file */
int rc = load_image(fd, file, user_data, data);
int rc = load_image(fd, file, user_data, data, padded_size);
if (rc <= 0) {
handle_id = ERR_FILE_ERROR;
} else {