[SECURITY PATCH 071/117] fs/fshelp: Catch impermissibly large block sizes in read helper

[Daniel Kiper]

From: Daniel Axtens <dja@axtens.net>

A fuzzed HFS+ filesystem had log2blocksize = 22. This gave
log2blocksize + GRUB_DISK_SECTOR_BITS = 31. 1 << 31 = 0x80000000,
which is -1 as an int. This caused some wacky behavior later on in
the function, leading to out-of-bounds writes on the destination buffer.

Catch log2blocksize + GRUB_DISK_SECTOR_BITS >= 31. We could be stricter,
but this is the minimum that will prevent integer size weirdness.

Signed-off-by: Daniel Axtens <dja@axtens.net>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
---
 grub-core/fs/fshelp.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/grub-core/fs/fshelp.c b/grub-core/fs/fshelp.c
index 4c902adf3..a2d0d297a 100644
--- a/grub-core/fs/fshelp.c
+++ b/grub-core/fs/fshelp.c
@@ -362,6 +362,18 @@ grub_fshelp_read_file (grub_disk_t disk, 
grub_fshelp_node_t node,
   grub_disk_addr_t i, blockcnt;
   int blocksize = 1 << (log2blocksize + GRUB_DISK_SECTOR_BITS);
 
+  /*
+   * Catch blatantly invalid log2blocksize. We could be a lot stricter, but
+   * this is the most permissive we can be before we start to see integer
+   * overflow/underflow issues.
+   */
+  if (log2blocksize + GRUB_DISK_SECTOR_BITS >= 31)
+    {
+      grub_error (GRUB_ERR_OUT_OF_RANGE,
+                 N_("blocksize too large"));
+      return -1;
+    }
+
   if (pos > filesize)
     {
       grub_error (GRUB_ERR_OUT_OF_RANGE,
-- 
2.11.0