lib/cmdline: avoid page fault in next_arg

lib/cmdline: avoid page fault in next_arg

An argument list like "arg=val arg2 \"" can trigger a page fault if the
page pointed by 'args[0xffffffff]' is not mapped and potential memory
corruption otherwise (unlikely but possible if the bogus address is mapped
and contents happen to match the ascii value of the quote character).

The fix is to ensure that we load 'args[i-1]' only when (i > 0).

Prior to this commit the following command would trigger an
unhandled page fault in the kernel:

root@(none):/linus/fs/fat# insmod ./fat.ko  "foo=bar \""
[   33.870507] BUG: unable to handle page fault for address: ffff888204252608
[   33.872180] #PF: supervisor read access in kernel mode
[   33.873414] #PF: error_code(0x0000) - not-present page
[   33.874650] PGD 4401067 P4D 4401067 PUD 0
[   33.875321] Oops: 0000 [#1] SMP DEBUG_PAGEALLOC PTI
[   33.876113] CPU: 16 PID: 399 Comm: insmod Not tainted 5.19.0-dbg-DEV #4
[   33.877193] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.0-debian-1.16.0-4 04/01/2014
[   33.878739] RIP: 0010:next_arg+0xd1/0x110
[   33.879399] Code: 22 75 1d 41 c6 04 01 00 41 80 f8 22 74 18 eb 35 4c 89 0e 45 31 d2 4c 89 cf 48 c7 02 00 00 00 00 41 80 f8 22 75 1f 41 8d 42 ff <41> 80 3c 01 22 75 14 41 c6 04 01 00 eb 0d 48 c7 02 00 00 00 00 41
[   33.882338] RSP: 0018:ffffc90001253d08 EFLAGS: 00010246
[   33.883174] RAX: 00000000ffffffff RBX: ffff888104252608 RCX: 0fc317bba1c1dd00
[   33.884311] RDX: ffffc90001253d40 RSI: ffffc90001253d48 RDI: ffff888104252609
[   33.885450] RBP: ffffc90001253d10 R08: 0000000000000022 R09: ffff888104252609
[   33.886595] R10: 0000000000000000 R11: ffffffff82c7ff20 R12: 0000000000000282
[   33.887748] R13: 00000000ffff8000 R14: 0000000000000000 R15: 0000000000007fff
[   33.888887] FS:  00007f04ec7432c0(0000) GS:ffff88813d300000(0000) knlGS:0000000000000000
[   33.890183] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   33.891111] CR2: ffff888204252608 CR3: 0000000100f36005 CR4: 0000000000170ee0
[   33.892241] Call Trace:
[   33.892641]  <TASK>
[   33.892989]  parse_args+0x8f/0x220
[   33.893538]  load_module+0x138b/0x15a0
[   33.894149]  ? prepare_coming_module+0x50/0x50
[   33.894879]  ? kernel_read_file_from_fd+0x5f/0x90
[   33.895639]  __se_sys_finit_module+0xce/0x130
[   33.896342]  __x64_sys_finit_module+0x1d/0x20
[   33.897042]  do_syscall_64+0x44/0xa0
[   33.897622]  entry_SYSCALL_64_after_hwframe+0x63/0xcd
[   33.898434] RIP: 0033:0x7f04ec85ef79
[   33.899009] Code: 48 8d 3d da db 0d 00 0f 05 eb a5 66 0f 1f 44 00 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 8b 0d c7 9e 0d 00 f7 d8 64 89 01 48
[   33.901912] RSP: 002b:00007fffae81bfe8 EFLAGS: 00000246 ORIG_RAX: 0000000000000139
[   33.903081] RAX: ffffffffffffffda RBX: 0000559c5f1d2640 RCX: 00007f04ec85ef79
[   33.904191] RDX: 0000000000000000 RSI: 0000559c5f1d12a0 RDI: 0000000000000003
[   33.905304] RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000000
[   33.906421] R10: 0000000000000003 R11: 0000000000000246 R12: 0000559c5f1d12a0
[   33.907526] R13: 0000000000000000 R14: 0000559c5f1d25f0 R15: 0000559c5f1d12a0
[   33.908631]  </TASK>
[   33.908986] Modules linked in: fat(+) [last unloaded: fat]
[   33.909843] CR2: ffff888204252608
[   33.910375] ---[ end trace 0000000000000000 ]---
[   33.911172] RIP: 0010:next_arg+0xd1/0x110
[   33.911796] Code: 22 75 1d 41 c6 04 01 00 41 80 f8 22 74 18 eb 35 4c 89 0e 45 31 d2 4c 89 cf 48 c7 02 00 00 00 00 41 80 f8 22 75 1f 41 8d 42 ff <41> 80 3c 01 22 75 14 41 c6 04 01 00 eb 0d 48 c7 02 00 00 00 00 41
[   33.914643] RSP: 0018:ffffc90001253d08 EFLAGS: 00010246
[   33.915446] RAX: 00000000ffffffff RBX: ffff888104252608 RCX: 0fc317bba1c1dd00
[   33.916544] RDX: ffffc90001253d40 RSI: ffffc90001253d48 RDI: ffff888104252609
[   33.917636] RBP: ffffc90001253d10 R08: 0000000000000022 R09: ffff888104252609
[   33.918727] R10: 0000000000000000 R11: ffffffff82c7ff20 R12: 0000000000000282
[   33.919821] R13: 00000000ffff8000 R14: 0000000000000000 R15: 0000000000007fff
[   33.920908] FS:  00007f04ec7432c0(0000) GS:ffff88813d300000(0000) knlGS:0000000000000000
[   33.922125] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   33.923017] CR2: ffff888204252608 CR3: 0000000100f36005 CR4: 0000000000170ee0
[   33.924098] Kernel panic - not syncing: Fatal exception
[   33.925776] Kernel Offset: disabled
[   33.926347] Rebooting in 10 seconds..

Link: https://lkml.kernel.org/r/20220728232434.1666488-1-neelnatu@google.com
Signed-off-by: Neel Natu <neelnatu@google.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>