lib/sbitmap: fix kernel crash observed when sbitmap depth is zero

We observed a kernel crash when the I/O scheduler allocates an sbitmap
for a hardware queue (hctx) that has no associated software queues (ctx),
and later attempts to free it. When no software queues are mapped to a
hardware queue, the sbitmap is initialized with a depth of zero. In such
cases, the sbitmap_init_node() function should set sb->alloc_hint to NULL.
However, if this is not done, sb->alloc_hint may contain garbage, and
calling sbitmap_free() will pass this invalid pointer to free_percpu(),
resulting in a kernel crash.

Example crash trace:
==================================================================
Kernel attempted to read user page (28) - exploit attempt? (uid: 0)
BUG: Kernel NULL pointer dereference on read at 0x00000028
Faulting instruction address: 0xc000000000708f88
Oops: Kernel access of bad area, sig: 11 [#1]
LE PAGE_SIZE=64K MMU=Radix  SMP NR_CPUS=2048 NUMA pSeries
[...]
CPU: 5 UID: 0 PID: 5491 Comm: mk_nullb_shared Kdump: loaded Tainted: G    B               6.16.0-rc5+ #294 VOLUNTARY
Tainted: [B]=BAD_PAGE
Hardware name: IBM,9043-MRX POWER10 (architected) 0x800200 0xf000006 of:IBM,FW1060.00 (NM1060_028) hv:phyp pSeries
[...]
NIP [c000000000708f88] free_percpu+0x144/0xba8
LR [c000000000708f84] free_percpu+0x140/0xba8
Call Trace:
    free_percpu+0x140/0xba8 (unreliable)
    kyber_exit_hctx+0x94/0x124
    blk_mq_exit_sched+0xe4/0x214
    elevator_exit+0xa8/0xf4
    elevator_switch+0x3b8/0x5d8
    elv_update_nr_hw_queues+0x14c/0x300
    blk_mq_update_nr_hw_queues+0x5cc/0x670
    nullb_update_nr_hw_queues+0x118/0x1f8 [null_blk]
    nullb_device_submit_queues_store+0xac/0x170 [null_blk]
    configfs_write_iter+0x1dc/0x2d0
    vfs_write+0x5b0/0x77c
    ksys_write+0xa0/0x180
    system_call_exception+0x1b0/0x4f0
    system_call_vectored_common+0x15c/0x2ec

If the sbitmap depth is zero, sb->alloc_hint memory is NOT allocated, but
the pointer is not explicitly set to NULL. Later, during sbitmap_free(),
the kernel attempts to free sb->alloc_hint, which is a per cpu pointer
variable, regardless of whether it was valid, leading to a crash.

This patch ensures that sb->alloc_hint is explicitly set to NULL in
sbitmap_init_node() when the requested depth is zero. This prevents
free_percpu() from freeing sb->alloc_hint and thus avoids the observed
crash.

Reviewed-by: Damien Le Moal <[email protected]>
Reviewed-by: Hannes Reinecke <[email protected]>
Signed-off-by: Nilay Shroff <[email protected]>
Reviewed-by: Yu Kuai <[email protected]>

Author: shroffni

lib/sbitmap.c

@@ -119,6 +119,7 @@ int sbitmap_init_node(struct sbitmap *sb, unsigned int depth, int shift,
 
 	if (depth == 0) {
 		sb->map = NULL;
+		sb->alloc_hint = NULL;
 		return 0;
 	}