TLDRVuln - Cybersecurity Vulnerabilities Made Simple

In the Linux kernel, the following vulnerability has been resolved:

powerpc/pci: Fix get_phb_number() locking

The recent change to get_phb_number() causes a DEBUG_ATOMIC_SLEEP

warning on some systems:

BUG: sleeping function called from invalid context at kernel/locking/mutex.c:580

in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid: 1, name: swapper

preempt_count: 1, expected: 0

RCU nest depth: 0, expected: 0

1 lock held by swapper/1:

#0: c157efb0 (hose_spinlock){+.+.}-{2:2}, at: pcibios_alloc_controller+0x64/0x220

Preemption disabled at:

[<00000000>] 0x0

CPU: 0 PID: 1 Comm: swapper Not tainted 5.19.0-yocto-standard+ #1

Call Trace:

[d101dc90] [c073b264] dump_stack_lvl+0x50/0x8c (unreliable)

[d101dcb0] [c0093b70] __might_resched+0x258/0x2a8

[d101dcd0] [c0d3e634] __mutex_lock+0x6c/0x6ec

[d101dd50] [c0a84174] of_alias_get_id+0x50/0xf4

[d101dd80] [c002ec78] pcibios_alloc_controller+0x1b8/0x220

[d101ddd0] [c140c9dc] pmac_pci_init+0x198/0x784

[d101de50] [c140852c] discover_phbs+0x30/0x4c

[d101de60] [c0007fd4] do_one_initcall+0x94/0x344

[d101ded0] [c1403b40] kernel_init_freeable+0x1a8/0x22c

[d101df10] [c00086e0] kernel_init+0x34/0x160

[d101df30] [c001b334] ret_from_kernel_thread+0x5c/0x64

This is because pcibios_alloc_controller() holds hose_spinlock but

of_alias_get_id() takes of_mutex which can sleep.

The hose_spinlock protects the phb_bitmap, and also the hose_list, but

it doesn't need to be held while get_phb_number() calls the OF routines,

because those are only looking up information in the device tree.

So fix it by having get_phb_number() take the hose_spinlock itself, only

where required, and then dropping the lock before returning.

pcibios_alloc_controller() then needs to take the lock again before the

list_add() but that's safe, the order of the list is not important.