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

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: 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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index eb34d204d4ee717f0cc3e8c640df1a4c10f1029a..e8d7d3c2bfcb8632bd702b697537b22c3c066e2f 100644 (file)
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -1524,7 +1524,7 @@ static inline void wb_dirty_limits(struct dirty_throttle_control *dtc)
         */
        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