[PATCH AUTOSEL 6.19-6.6] scsi: ufs: core: Fix shift out of bounds when MAXQ=32

Sasha Levin
2026-03-05T15:37:28Z

From: wangshuaiwei <wangshuaiwei1@xiaomi.com>

[ Upstream commit 2f38fd99c0004676d835ae96ac4f3b54edc02c82 ]

According to JESD223F, the maximum number of queues (MAXQ) is 32. When MCQ
is enabled and ESI is disabled, nr_hw_queues=32 causes a shift overflow
problem.

Fix this by using 64-bit intermediate values to handle the nr_hw_queues=32
case safely.

Signed-off-by: wangshuaiwei <wangshuaiwei1@xiaomi.com>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Link: https://patch.msgid.link/20260224063228.50112-1-wangshuaiwei1@xiaomi.com
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---

LLM Generated explanations, may be completely bogus:

## Analysis

### What the commit fixes

This is a classic **undefined behavior (UB) / shift out of bounds** bug.
The expression `(1U << hba->nr_hw_queues) - 1` uses a 32-bit unsigned
integer literal `1U`. When `hba->nr_hw_queues == 32` (which is the
maximum allowed by the JESD223F UFS specification), the expression `1U
<< 32` is undefined behavior in C — shifting a 32-bit value by 32 or
more positions is explicitly undefined per the C standard.

The fix changes `1U` to `1ULL` (64-bit), making the shift well-defined
for values up to 63.

### Bug mechanism and impact

- **Variable:** `outstanding_cqs` is declared as `unsigned long` (64-bit
on 64-bit platforms)
- **Context:** This is a fallback path in the interrupt handler
`ufshcd_handle_mcq_cq_events()` — executed when
`ufshcd_vops_get_outstanding_cqs()` fails (vendor-specific register
not available)
- **Trigger:** Hardware with MAXQ=32 (the maximum allowed by UFS spec)
- **Consequence:** On such hardware, the undefined behavior could result
in `outstanding_cqs` being set to 0 instead of the intended bitmask of
all 1s (0xFFFFFFFF). This would mean **no completion queues get
serviced**, potentially causing I/O hangs or lost completions — a
severe storage subsystem issue.

### Stable kernel criteria assessment

1. **Obviously correct and tested:** Yes — a single-character change
(`U` → `ULL`), reviewed by Bart Van Assche (UFS maintainer). The fix
is trivially correct.
2. **Fixes a real bug:** Yes — undefined behavior that can cause I/O
failures on hardware with 32 queues.
3. **Important issue:** Yes — storage I/O hangs are critical. UFS is the
standard storage interface for mobile devices.
4. **Small and contained:** Yes — a single line change, single character
modification.
5. **No new features:** Correct — pure bug fix.

### Risk assessment

**Risk: Extremely low.** This is a one-character change from `1U` to
`1ULL`. It cannot introduce regressions — on hardware with fewer than 32
queues, the behavior is identical. On hardware with exactly 32 queues,
it fixes the undefined behavior.

### Affected versions

The buggy code was introduced in commit `f87b2c41822aa` ("scsi: ufs:
mcq: Add completion support of a CQE") which landed in v6.3 (merged
January 2023). All stable trees from 6.3 onward that include MCQ support
are affected.

### Verification

- **git blame** confirmed the buggy line `(1U << hba->nr_hw_queues) - 1`
originates from commit `f87b2c41822aa` (January 2023)
- **Code reading** confirmed `outstanding_cqs` is `unsigned long` and
`nr_hw_queues` is `unsigned int`, verifying the type mismatch concern
- **Read `ufs-mcq.c:174`** confirmed `hba_maxq` is derived from
`FIELD_GET(MAX_QUEUE_SUP, ...)` + 1, and per JESD223F the max is 32,
confirming `nr_hw_queues=32` is a valid hardware configuration
- **Read `ufs-mcq.c:193-219`** confirmed `hba->nr_hw_queues` is set to
the total number of queues which can reach `hba_maxq` (up to 32)
- **Reviewed-by: Bart Van Assche** — UFS subsystem expert confirms the
fix
- The commit applies to a single file with a trivial one-character
change

This is a textbook stable backport candidate: a one-character fix for
undefined behavior in a storage driver interrupt handler, with potential
for I/O hangs on compliant hardware. Minimal risk, clear correctness,
important subsystem.

**YES**

drivers/ufs/core/ufshcd.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/ufs/core/ufshcd.c b/drivers/ufs/core/ufshcd.c
index 27d53a044dbad..f65b0aeef6dde 100644
--- a/drivers/ufs/core/ufshcd.c
+++ b/drivers/ufs/core/ufshcd.c
@@ -7094,7 +7094,7 @@ static irqreturn_t ufshcd_handle_mcq_cq_events(struct ufs_hba *hba)

ret = ufshcd_vops_get_outstanding_cqs(hba, &outstanding_cqs);
if (ret)
- outstanding_cqs = (1U << hba->nr_hw_queues) - 1;
+ outstanding_cqs = (1ULL << hba->nr_hw_queues) - 1;

/* Exclude the poll queues */
nr_queues = hba->nr_hw_queues - hba->nr_queues[HCTX_TYPE_POLL];
--
2.51.0