ext4: fix BUG in ext4_mb_new_inode_pa() due to overflow

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

ext4: fix BUG in ext4_mb_new_inode_pa() due to overflow

When we calculate the end position of ext4_free_extent, this position may

be exactly where ext4_lblk_t (i.e. uint) overflows. For example, if

ac_g_ex.fe_logical is 4294965248 and ac_orig_goal_len is 2048, then the

computed end is 0x100000000, which is 0. If ac->ac_o_ex.fe_logical is not

the first case of adjusting the best extent, that is, new_bex_end > 0, the

following BUG_ON will be triggered:

=========================================================

kernel BUG at fs/ext4/mballoc.c:5116!

invalid opcode: 0000 [#1] PREEMPT SMP PTI

CPU: 3 PID: 673 Comm: xfs_io Tainted: G E 6.5.0-rc1+ #279

RIP: 0010:ext4_mb_new_inode_pa+0xc5/0x430

Call Trace:

<TASK>

ext4_mb_use_best_found+0x203/0x2f0

ext4_mb_try_best_found+0x163/0x240

ext4_mb_regular_allocator+0x158/0x1550

ext4_mb_new_blocks+0x86a/0xe10

ext4_ext_map_blocks+0xb0c/0x13a0

ext4_map_blocks+0x2cd/0x8f0

ext4_iomap_begin+0x27b/0x400

iomap_iter+0x222/0x3d0

__iomap_dio_rw+0x243/0xcb0

iomap_dio_rw+0x16/0x80

=========================================================

A simple reproducer demonstrating the problem:

mkfs.ext4 -F /dev/sda -b 4096 100M

mount /dev/sda /tmp/test

fallocate -l1M /tmp/test/tmp

fallocate -l10M /tmp/test/file

fallocate -i -o 1M -l16777203M /tmp/test/file

fsstress -d /tmp/test -l 0 -n 100000 -p 8 &

sleep 10 && killall -9 fsstress

rm -f /tmp/test/tmp

xfs_io -c "open -ad /tmp/test/file" -c "pwrite -S 0xff 0 8192"

We simply refactor the logic for adjusting the best extent by adding

a temporary ext4_free_extent ex and use extent_logical_end() to avoid

overflow, which also simplifies the code.