ext4: allow ext4_get_group_info() to fail

Vulnerability Description

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.

References

• https://git.kernel.org/stable/c/100c0ad6c04597fefeaaba2bb1827cc015d95067
• https://git.kernel.org/stable/c/620a3c28221bb219b81bc0bffd065cc187494302
• https://git.kernel.org/stable/c/b4319e457d6e3fb33e443efeaf4634fc36e8a9ed
• https://git.kernel.org/stable/c/31668cebf45adfb6283e465e641c4f5a21b07afa
• https://git.kernel.org/stable/c/5354b2af34064a4579be8bc0e2f15a7b70f14b5f