Re: [PATCH 1/2] reboot: fix overflow parsing reboot cpu number

[Kees Cook]

On Fri, Oct 16, 2020 at 08:09:06PM +0200, Matteo Croce wrote:
> From: Matteo Croce <mcroce@xxxxxxxxxxxxx>
> 
> Limit the CPU number to num_possible_cpus(), because setting it
> to a value lower than INT_MAX but higher than NR_CPUS produces the
> following error on reboot and shutdown:
> 
>     BUG: unable to handle page fault for address: ffffffff90ab1bb0
>     #PF: supervisor read access in kernel mode
>     #PF: error_code(0x0000) - not-present page
>     PGD 1c09067 P4D 1c09067 PUD 1c0a063 PMD 0
>     Oops: 0000 [#1] SMP
>     CPU: 1 PID: 1 Comm: systemd-shutdow Not tainted 5.9.0-rc8-kvm #110
>     Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.13.0-2.fc32 04/01/2014
>     RIP: 0010:migrate_to_reboot_cpu+0xe/0x60
>     Code: ea ea 00 48 89 fa 48 c7 c7 30 57 f1 81 e9 fa ef ff ff 66 2e 0f 1f 84 00 00 00 00 00 53 8b 1d d5 ea ea 00 e8 14 33 fe ff 89 da <48> 0f a3 15 ea fc bd 00 48 89 d0 73 29 89 c2 c1 e8 06 65 48 8b 3c
>     RSP: 0018:ffffc90000013e08 EFLAGS: 00010246
>     RAX: ffff88801f0a0000 RBX: 0000000077359400 RCX: 0000000000000000
>     RDX: 0000000077359400 RSI: 0000000000000002 RDI: ffffffff81c199e0
>     RBP: ffffffff81c1e3c0 R08: ffff88801f41f000 R09: ffffffff81c1e348
>     R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
>     R13: 00007f32bedf8830 R14: 00000000fee1dead R15: 0000000000000000
>     FS:  00007f32bedf8980(0000) GS:ffff88801f480000(0000) knlGS:0000000000000000
>     CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>     CR2: ffffffff90ab1bb0 CR3: 000000001d057000 CR4: 00000000000006a0
>     DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
>     DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
>     Call Trace:
>     __do_sys_reboot.cold+0x34/0x5b
>     ? vfs_writev+0x92/0xc0
>     ? do_writev+0x52/0xd0
>     do_syscall_64+0x2d/0x40
>     entry_SYSCALL_64_after_hwframe+0x44/0xa9
>     RIP: 0033:0x7f32bfaaecd3
>     Code: 64 89 01 48 83 c8 ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 89 fa be 69 19 12 28 bf ad de e1 fe b8 a9 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 05 c3 0f 1f 40 00 48 8b 15 89 81 0c 00 f7 d8
>     RSP: 002b:00007fff6265fb58 EFLAGS: 00000202 ORIG_RAX: 00000000000000a9
>     RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007f32bfaaecd3
>     RDX: 0000000001234567 RSI: 0000000028121969 RDI: 00000000fee1dead
>     RBP: 0000000000000000 R08: 0000000000008020 R09: 00007fff6265ef60
>     R10: 00007f32bedf8830 R11: 0000000000000202 R12: 0000000000000000
>     R13: 0000557bba2c51c0 R14: 0000000000000000 R15: 00007fff6265fbc8
>     CR2: ffffffff90ab1bb0
>     ---[ end trace b813e80157136563 ]---
>     RIP: 0010:migrate_to_reboot_cpu+0xe/0x60
>     Code: ea ea 00 48 89 fa 48 c7 c7 30 57 f1 81 e9 fa ef ff ff 66 2e 0f 1f 84 00 00 00 00 00 53 8b 1d d5 ea ea 00 e8 14 33 fe ff 89 da <48> 0f a3 15 ea fc bd 00 48 89 d0 73 29 89 c2 c1 e8 06 65 48 8b 3c
>     RSP: 0018:ffffc90000013e08 EFLAGS: 00010246
>     RAX: ffff88801f0a0000 RBX: 0000000077359400 RCX: 0000000000000000
>     RDX: 0000000077359400 RSI: 0000000000000002 RDI: ffffffff81c199e0
>     RBP: ffffffff81c1e3c0 R08: ffff88801f41f000 R09: ffffffff81c1e348
>     R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
>     R13: 00007f32bedf8830 R14: 00000000fee1dead R15: 0000000000000000
>     FS:  00007f32bedf8980(0000) GS:ffff88801f480000(0000) knlGS:0000000000000000
>     CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>     CR2: ffffffff90ab1bb0 CR3: 000000001d057000 CR4: 00000000000006a0
>     DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
>     DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
>     Kernel panic - not syncing: Attempted to kill init! exitcode=0x00000009
>     Kernel Offset: disabled
>     ---[ end Kernel panic - not syncing: Attempted to kill init! exitcode=0x00000009 ]---
> 
> Fixes: 1b3a5d02ee07 ("reboot: move arch/x86 reboot= handling to generic kernel")
> Signed-off-by: Matteo Croce <mcroce@xxxxxxxxxxxxx>

Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>

-- 
Kees Cook