blk-throttle: fix divide-by-zero on legacy iops limit of 0

From: Tao Cui <[email protected]>

Writing a multiple of 2^32 (e.g. 4294967296) to a legacy cgroup v1 throttle iops file (blkio.throttle.{read,write}_iops_device) silently truncates to 0: tg_set_conf() stores the sscanf-parsed u64 value into an unsigned int field with no clamping. The cgroup v2 path, tg_set_limit(), already clamps the same kind of value with min_t(u64, val, UINT_MAX), but the legacy path never did. Note that the "!v -> U64_MAX" mapping only catches an explicit zero and does not catch a value that truncates to zero.

With iops stored as 0, tg_update_has_rules() sets has_rules_iops[] and the next IO reaches tg_within_iops_limit(), which computes

     jiffy_wait = max(jiffy_wait, HZ / iops_limit + 1);

triggering a divide-by-zero oops.

Fix it in tg_set_conf() by clamping the value to UINT_MAX, consistent with tg_set_limit(). This closes the truncation root cause: with 0 no longer reachable as a stored limit, the HZ / iops_limit divide is never hit.

Signed-off-by Tao Cui <[email protected]>

Reviewed-by Yu Kuai <[email protected]>

---
Changes in v5:
- Rebase onto current linux-next head (no code change, context shifted only).
- Add the Reviewed-by tag collected on v4.
- Link to v4: https://lore.kernel.org/r/[email protected]

Changes in v4:
- Drop the defensive "iops_limit == 0" check in tg_dispatch_iops_time():
with the tg_set_conf() clamp in place, 0 can never be stored as a limit, so the runtime check only guards an unreachable state. (Yu Kuai)
- Drop the Fixes: tag: the unclamped write -- and the iops=0 behavior it can produce (calculate_io_allowed() returns 0, so no IO is issued) -- long predates the commit that added the HZ / iops_limit divide, so attributing it there was incorrect. (Yu Kuai)

Changes in v3:
- Drop the (u64) cast on UINT_MAX: the kernel's type-checked min() accepts two unsigned types of different width (both >= 4 bytes), so min(v, UINT_MAX) compiles clean. (David Laight)

Changes in v2:
- Use a "void *field" local for the config write so the assignment reads *(u64 *)field / *(unsigned int *)field instead of the (type *)((void *)tg + of_cft(of)->private) casts.
- Use min(v, UINT_MAX) instead of min_t(u64, v, UINT_MAX).

---
  block/blk-throttle.c | 6 ++++--
  1 file changed, 4 insertions(+), 2 deletions(-)

block/blk-throttle.c +4 -2

index ffc3b70065d4..97ad7959d006 100644
@@ -1383,10 +1383,12 @@ static ssize_t tg_set_conf(struct kernfs_open_file *of,
  	tg = blkg_to_tg(ctx.blkg);
  	tg_update_carryover(tg);
 
+	void *field = (void *)tg + of_cft(of)->private;
+
  	if (is_u64)
-		*(u64 *)((void *)tg + of_cft(of)->private) = v;
+		*(u64 *)field = v;
  	else
-		*(unsigned int *)((void *)tg + of_cft(of)->private) = v;
+		*(unsigned int *)field = min(v, UINT_MAX);
 
  	tg_conf_updated(tg, false);
  	ret = 0;

Hi,

Quoted 18 lines · Tao Cui

From: Tao Cui <[email protected]>

Writing a multiple of 2^32 (e.g. 4294967296) to a legacy cgroup v1 throttle iops file (blkio.throttle.{read,write}_iops_device) silently truncates to 0: tg_set_conf() stores the sscanf-parsed u64 value into an unsigned int field with no clamping. The cgroup v2 path, tg_set_limit(), already clamps the same kind of value with min_t(u64, val, UINT_MAX), but the legacy path never did. Note that the "!v -> U64_MAX" mapping only catches an explicit zero and does not catch a value that truncates to zero.

With iops stored as 0, tg_update_has_rules() sets has_rules_iops[] and the next IO reaches tg_within_iops_limit(), which computes

     jiffy_wait = max(jiffy_wait, HZ / iops_limit + 1);

triggering a divide-by-zero oops.

Quoted 7 lines

Fix it in tg_set_conf() by clamping the value to UINT_MAX, consistent with tg_set_limit(). This closes the truncation root cause: with 0 no longer reachable as a stored limit, the HZ / iops_limit divide is never hit.

Signed-off-by Tao Cui <[email protected]>

Reviewed-by Yu Kuai <[email protected]>

This tag is wrong, please use [email protected].

Quoted 49 lines

---
Changes in v5:
- Rebase onto current linux-next head (no code change, context shifted only).
- Add the Reviewed-by tag collected on v4.
- Link to v4: https://lore.kernel.org/r/[email protected]

Changes in v4:
- Drop the defensive "iops_limit == 0" check in tg_dispatch_iops_time():
with the tg_set_conf() clamp in place, 0 can never be stored as a limit, so the runtime check only guards an unreachable state. (Yu Kuai)
- Drop the Fixes: tag: the unclamped write -- and the iops=0 behavior it can produce (calculate_io_allowed() returns 0, so no IO is issued) -- long predates the commit that added the HZ / iops_limit divide, so attributing it there was incorrect. (Yu Kuai)

Changes in v3:
- Drop the (u64) cast on UINT_MAX: the kernel's type-checked min() accepts two unsigned types of different width (both >= 4 bytes), so min(v, UINT_MAX) compiles clean. (David Laight)

Changes in v2:
- Use a "void *field" local for the config write so the assignment reads *(u64 *)field / *(unsigned int *)field instead of the (type *)((void *)tg + of_cft(of)->private) casts.
- Use min(v, UINT_MAX) instead of min_t(u64, v, UINT_MAX).

---
  block/blk-throttle.c | 6 ++++--
  1 file changed, 4 insertions(+), 2 deletions(-)

block/blk-throttle.c +4 -2

index ffc3b70065d4..97ad7959d006 100644
@@ -1383,10 +1383,12 @@ static ssize_t tg_set_conf(struct kernfs_open_file *of,
  	tg = blkg_to_tg(ctx.blkg);
  	tg_update_carryover(tg);
 
+	void *field = (void *)tg + of_cft(of)->private;
+
  	if (is_u64)
-		*(u64 *)((void *)tg + of_cft(of)->private) = v;
+		*(u64 *)field = v;
  	else
-		*(unsigned int *)((void *)tg + of_cft(of)->private) = v;
+		*(unsigned int *)field = min(v, UINT_MAX);
 
  	tg_conf_updated(tg, false);
  	ret = 0;