[PATCH] reboot: fix parsing of reboot cpu number

[PATCH] reboot: fix parsing of reboot cpu number

[Matteo Croce]

From: Matteo Croce <mcroce@microsoft.com>

The kernel cmdline reboot= argument allows to specify the CPU used
for rebooting, with the syntax `s####` among the other flags, e.g.

  reboot=soft,s4
  reboot=warm,s31,force

In the early days the parsing was done with simple_strtoul(), later
deprecated in favor of the safer kstrtoint() which handles overflow.

But kstrtoint() returns -EINVAL if there are non-digit characters
in a string, so if this flag is not the last given, it's silently
ignored as well as the subsequent ones.

To fix it, use _parse_integer() which still handles overflow, but
restores the old behaviour of parsing until a non-digit character
is found.

While at it, 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")
Fixes: 616feab75397 ("kernel/reboot.c: convert simple_strtoul to kstrtoint")
Signed-off-by: Matteo Croce <mcroce@microsoft.com>
---
 kernel/reboot.c | 30 ++++++++++++++++++------------
 1 file changed, 18 insertions(+), 12 deletions(-)

diff --git a/kernel/reboot.c b/kernel/reboot.c
index e7b78d5ae1ab..79e5d4522d0e 100644
--- a/kernel/reboot.c
+++ b/kernel/reboot.c
@@ -18,6 +18,8 @@
 #include <linux/syscore_ops.h>
 #include <linux/uaccess.h>
 
+#include "../lib/kstrtox.h"
+
 /*
  * this indicates whether you can reboot with ctrl-alt-del: the default is yes
  */
@@ -552,19 +554,23 @@ static int __init reboot_setup(char *str)
 
 		case 's':
 		{
-			int rc;
-
-			if (isdigit(*(str+1))) {
-				rc = kstrtoint(str+1, 0, &reboot_cpu);
-				if (rc)
-					return rc;
-			} else if (str[1] == 'm' && str[2] == 'p' &&
-				   isdigit(*(str+3))) {
-				rc = kstrtoint(str+3, 0, &reboot_cpu);
-				if (rc)
-					return rc;
-			} else
+			unsigned long long number;
+			unsigned int rc;
+
+			/*
+			 * reboot_cpu is s[mp]#### with #### being the processor
+			 * to be used for rebooting. Skip 's' or 'smp' prefix.
+			 */
+			str += str[1] == 'm' && str[2] == 'p' ? 3 : 1;
+
+			if (isdigit(str[0])) {
+				rc = _parse_integer(str, 10, &number);
+				if (rc & KSTRTOX_OVERFLOW || number > num_possible_cpus())
+					return -ERANGE;
+				reboot_cpu = number;
+			} else {
 				*mode = REBOOT_SOFT;
+			}
 			break;
 		}
 		case 'g':
-- 
2.26.2

Re: [PATCH] reboot: fix parsing of reboot cpu number

[Petr Mladek]

On Wed 2020-10-14 23:27:46, Matteo Croce wrote:
> From: Matteo Croce <mcroce@microsoft.com>
> 
> The kernel cmdline reboot= argument allows to specify the CPU used
> for rebooting, with the syntax `s####` among the other flags, e.g.
> 
>   reboot=soft,s4
>   reboot=warm,s31,force
> 
> In the early days the parsing was done with simple_strtoul(), later
> deprecated in favor of the safer kstrtoint() which handles overflow.
> 
> But kstrtoint() returns -EINVAL if there are non-digit characters
> in a string, so if this flag is not the last given, it's silently
> ignored as well as the subsequent ones.
> 
> To fix it, use _parse_integer() which still handles overflow, but
> restores the old behaviour of parsing until a non-digit character
> is found.

Hmm, _parse_integer() is an internal function. And even the comment
says "Don't you dare use this function."

I guess the it is because the base must be hardcoded. And
KSTRTOX_OVERFLOW bit must be handled.

I suggest to go back to simple_strtoul(). It is not longer obsolete.
It still exists because it is needed for exactly this purpose,
see the comment in include/linux/kernel.h

The potentional overflow is not a big deal. The result will be
that the system will reboot on another rCPU than expected. But
it might happen also with any typo.

> While at it, 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:

Great catch! Please, fix this in a separate patch.

Best Regards,
Petr

Re: [PATCH] reboot: fix parsing of reboot cpu number

[Matteo Croce]

On Fri, Oct 16, 2020 at 2:20 PM Petr Mladek <pmladek@suse.com> wrote:
>
> On Wed 2020-10-14 23:27:46, Matteo Croce wrote:
> > From: Matteo Croce <mcroce@microsoft.com>
> >
> > The kernel cmdline reboot= argument allows to specify the CPU used
> > for rebooting, with the syntax `s####` among the other flags, e.g.
> >
> >   reboot=soft,s4
> >   reboot=warm,s31,force
> >
> > In the early days the parsing was done with simple_strtoul(), later
> > deprecated in favor of the safer kstrtoint() which handles overflow.
> >
> > But kstrtoint() returns -EINVAL if there are non-digit characters
> > in a string, so if this flag is not the last given, it's silently
> > ignored as well as the subsequent ones.
> >
> > To fix it, use _parse_integer() which still handles overflow, but
> > restores the old behaviour of parsing until a non-digit character
> > is found.
>
> Hmm, _parse_integer() is an internal function. And even the comment
> says "Don't you dare use this function."
>
> I guess the it is because the base must be hardcoded. And
> KSTRTOX_OVERFLOW bit must be handled.
>

I know, but it's the only function that I know which has the same
behaviour of simple_strtoul(). Other than sscanf, which uses
simple_strtoul().
I didn't know that simple_strtoul() is no longer obsolete, I will
revert to it then.

> I suggest to go back to simple_strtoul(). It is not longer obsolete.
> It still exists because it is needed for exactly this purpose,
> see the comment in include/linux/kernel.h
>
> The potentional overflow is not a big deal. The result will be
> that the system will reboot on another rCPU than expected. But
> it might happen also with any typo.
>
> > While at it, 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:
>
> Great catch! Please, fix this in a separate patch.
>

Ok, thanks!

-- 
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