The patch titled Subject: mm/writeback: fix possible divide-by-zero in wb_dirty_limits(), again has been added to the -mm mm-hotfixes-unstable branch. Its filename is mm-writeback-fix-possible-divide-by-zero-in-wb_dirty_limits-again.patch

This patch will shortly appear at https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches...

This patch will later appear in the mm-hotfixes-unstable branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------ From: "Zach O'Keefe" zokeefe@google.com Subject: mm/writeback: fix possible divide-by-zero in wb_dirty_limits(), again Date: Thu, 18 Jan 2024 10:19:53 -0800

(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: f6789593d5ce ("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 ---

mm/page-writeback.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/mm/page-writeback.c~mm-writeback-fix-possible-divide-by-zero-in-wb_dirty_limits-again +++ a/mm/page-writeback.c @@ -1638,7 +1638,7 @@ static inline void wb_dirty_limits(struc */ dtc->wb_thresh = __wb_calc_thresh(dtc); dtc->wb_bg_thresh = dtc->thresh ? - div_u64((u64)dtc->wb_thresh * dtc->bg_thresh, dtc->thresh) : 0; + div64_u64(dtc->wb_thresh * dtc->bg_thresh, dtc->thresh) : 0;

/* * In order to avoid the stacked BDI deadlock we need _

Patches currently in -mm which might be from zokeefe@google.com are

mm-writeback-fix-possible-divide-by-zero-in-wb_dirty_limits-again.patch