mm/writeback: fix possible divide-by-zero in wb_dirty_limits(), again

stable inclusion
from stable-v5.10.210
commit 81e7d2530d458548b90a5c5e76b77ad5e5d1c0df
category: bugfix
bugzilla: https://gitee.com/src-openeuler/kernel/issues/I9E2EC
CVE: CVE-2024-26720

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=81e7d2530d458548b90a5c5e76b77ad5e5d1c0df

--------------------------------

commit 9319b647902cbd5cc884ac08a8a6d54ce111fc78 upstream.

(struct dirty_throttle_control *)->thresh is an unsigned long, but is
passed as the u32 divisor argument to div_u64().  On architectures where
unsigned long is 64 bytes, the argument will be implicitly truncated.

Use div64_u64() instead of div_u64() so that the value used in the "is
this a safe division" check is the same as the divisor.

Also, remove redundant cast of the numerator to u64, as that should happen
implicitly.

This would be difficult to exploit in memcg domain, given the ratio-based
arithmetic domain_drity_limits() uses, but is much easier in global
writeback domain with a BDI_CAP_STRICTLIMIT-backing device, using e.g.
vm.dirty_bytes=(1<<32)*PAGE_SIZE so that dtc->thresh == (1<<32)

Link: https://lkml.kernel.org/r/20240118181954.1415197-1-zokeefe@google.com


Fixes: f6789593 ("mm/page-writeback.c: fix divide by zero in bdi_dirty_limits()")
Signed-off-by: Zach O'Keefe <zokeefe@google.com>
Cc: Maxim Patlasov <MPatlasov@parallels.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Ze Zuo <zuoze1@huawei.com>