CVE-2023-54121: btrfs: fix incorrect splitting in btrfs_drop_extent_map_range

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

btrfs: fix incorrect splitting in btrfsdropextentmaprange

In production we were seeing a variety of WARNON()'s in the extentmap code, specifically in btrfsdropextentmaprange() when we have to call addextentmapping() for our second split.

Consider the following extent map layout

PINNED [0 16K) [32K, 48K)

and then we call btrfsdropextentmaprange for [0, 36K), with skippinned == true. The initial loop will have

start = 0 end = 36K len = 36K

we will find the [0, 16k) extent, but since we are pinned we will skip it, which has this code

start = emend; if (end != (u64)-1) len = start + len - emend;

emend here is 16K, so now the values are

start = 16K len = 16K + 36K - 16K = 36K

len should instead be 20K. This is a problem when we find the next extent at [32K, 48K), we need to split this extent to leave [36K, 48k), however the code for the split looks like this

split->start = start + len; split->len = emend - (start + len);

In this case we have

emend = 48K split->start = 16K + 36K // this should be 16K + 20K split->len = 48K - (16K + 36K) // this overflows as 16K + 36K is 52K

and now we have an invalid extentmap in the tree that potentially overlaps other entries in the extent map. Even in the non-overlapping case we will have split->start set improperly, which will cause problems with any block related calculations.

We don't actually need len in this loop, we can simply use end as our end point, and only adjust start up when we find a pinned extent we need to skip.

Adjust the logic to do this, which keeps us from inserting an invalid extent map.

We only skippinned in the relocation case, so this is relatively rare, except in the case where you are running relocation a lot, which can happen with auto relocation on.