parisc: Fix data TLB miss in sba_unmap_sg

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

parisc: Fix data TLB miss in sba_unmap_sg

Rolf Eike Beer reported the following bug:

[1274934.746891] Bad Address (null pointer deref?): Code=15 (Data TLB miss fault) at addr 0000004140000018

[1274934.746891] CPU: 3 PID: 5549 Comm: cmake Not tainted 5.15.4-gentoo-parisc64 #4

[1274934.746891] Hardware name: 9000/785/C8000

[1274934.746891]

[1274934.746891] YZrvWESTHLNXBCVMcbcbcbcbOGFRQPDI

[1274934.746891] PSW: 00001000000001001111111000001110 Not tainted

[1274934.746891] r00-03 000000ff0804fe0e 0000000040bc9bc0 00000000406760e4 0000004140000000

[1274934.746891] r04-07 0000000040b693c0 0000004140000000 000000004a2b08b0 0000000000000001

[1274934.746891] r08-11 0000000041f98810 0000000000000000 000000004a0a7000 0000000000000001

[1274934.746891] r12-15 0000000040bddbc0 0000000040c0cbc0 0000000040bddbc0 0000000040bddbc0

[1274934.746891] r16-19 0000000040bde3c0 0000000040bddbc0 0000000040bde3c0 0000000000000007

[1274934.746891] r20-23 0000000000000006 000000004a368950 0000000000000000 0000000000000001

[1274934.746891] r24-27 0000000000001fff 000000000800000e 000000004a1710f0 0000000040b693c0

[1274934.746891] r28-31 0000000000000001 0000000041f988b0 0000000041f98840 000000004a171118

[1274934.746891] sr00-03 00000000066e5800 0000000000000000 0000000000000000 00000000066e5800

[1274934.746891] sr04-07 0000000000000000 0000000000000000 0000000000000000 0000000000000000

[1274934.746891]

[1274934.746891] IASQ: 0000000000000000 0000000000000000 IAOQ: 00000000406760e8 00000000406760ec

[1274934.746891] IIR: 48780030 ISR: 0000000000000000 IOR: 0000004140000018

[1274934.746891] CPU: 3 CR30: 00000040e3a9c000 CR31: ffffffffffffffff

[1274934.746891] ORIG_R28: 0000000040acdd58

[1274934.746891] IAOQ[0]: sba_unmap_sg+0xb0/0x118

[1274934.746891] IAOQ[1]: sba_unmap_sg+0xb4/0x118

[1274934.746891] RP(r2): sba_unmap_sg+0xac/0x118

[1274934.746891] Backtrace:

[1274934.746891] [<00000000402740cc>] dma_unmap_sg_attrs+0x6c/0x70

[1274934.746891] [<000000004074d6bc>] scsi_dma_unmap+0x54/0x60

[1274934.746891] [<00000000407a3488>] mptscsih_io_done+0x150/0xd70

[1274934.746891] [<0000000040798600>] mpt_interrupt+0x168/0xa68

[1274934.746891] [<0000000040255a48>] __handle_irq_event_percpu+0xc8/0x278

[1274934.746891] [<0000000040255c34>] handle_irq_event_percpu+0x3c/0xd8

[1274934.746891] [<000000004025ecb4>] handle_percpu_irq+0xb4/0xf0

[1274934.746891] [<00000000402548e0>] generic_handle_irq+0x50/0x70

[1274934.746891] [<000000004019a254>] call_on_stack+0x18/0x24

[1274934.746891]

[1274934.746891] Kernel panic - not syncing: Bad Address (null pointer deref?)

The bug is caused by overrunning the sglist and incorrectly testing

sg_dma_len(sglist) before nents. Normally this doesn't cause a crash,

but in this case sglist crossed a page boundary. This occurs in the

following code:

while (sg_dma_len(sglist) && nents--) {

The fix is simply to test nents first and move the decrement of nents

into the loop.