ext4: allow ext4_get_group_info() to fail

Overview

In the Linux kernel, the following vulnerability has been resolved:

ext4: allow ext4_get_group_info() to fail

Previously, ext4_get_group_info() would treat an invalid group number as BUG(), since in theory it should never happen. However, if a malicious attaker (or fuzzer) modifies the superblock via the block device while it is the file system is mounted, it is possible for s_first_data_block to get set to a very large number. In that case, when calculating the block group of some block number (such as the starting block of a preallocation region), could result in an underflow and very large block group number. Then the BUG_ON check in ext4_get_group_info() would fire, resutling in a denial of service attack that can be triggered by root or someone with write access to the block device.

For a quality of implementation perspective, it's best that even if the system administrator does something that they shouldn't, that it will not trigger a BUG. So instead of BUG'ing, ext4_get_group_info() will call ext4_error and return NULL. We also add fallback code in all of the callers of ext4_get_group_info() that it might NULL.

Also, since ext4_get_group_info() was already borderline to be an inline function, un-inline it. The results in a next reduction of the compiled text size of ext4 by roughly 2k.

Affected

• Linux >= 2f2e09eb15849562aede80ed007658e4504ded26 < 100c0ad6c04597fefeaaba2bb1827cc015d95067
• Linux >= 2f2e09eb15849562aede80ed007658e4504ded26 < 620a3c28221bb219b81bc0bffd065cc187494302
• Linux >= 2f2e09eb15849562aede80ed007658e4504ded26 < b4319e457d6e3fb33e443efeaf4634fc36e8a9ed
• Linux >= 2f2e09eb15849562aede80ed007658e4504ded26 < 31668cebf45adfb6283e465e641c4f5a21b07afa
• Linux >= 2f2e09eb15849562aede80ed007658e4504ded26 < 5354b2af34064a4579be8bc0e2f15a7b70f14b5f
• Linux 3.11

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.