On Wed, Apr 03, 2019 at 04:34:09PM +0100, Will Deacon wrote:
> Commit 32a5ad9c2285 ("sysctl: handle overflow for file-max") hooked
> up min/max values for the file-max sysctl parameter via the .extra1
> and .extra2 fields in the corresponding struct ctl_table entry.
>
> Unfortunately, the minimum value points at the global 'zero' variable,
> which is an int. This results in a KASAN splat when accessed as a long
> by proc_doulongvec_minmax on 64-bit architectures:
>
> | BUG: KASAN: global-out-of-bounds in
> __do_proc_doulongvec_minmax+0x5d8/0x6a0
> | Read of size 8 at addr ffff2000133d1c20 by task systemd/1
> |
> | CPU: 0 PID: 1 Comm: systemd Not tainted 5.1.0-rc3-00012-g40b114779944 #2
> | Hardware name: linux,dummy-virt (DT)
> | Call trace:
> | dump_backtrace+0x0/0x228
> | show_stack+0x14/0x20
> | dump_stack+0xe8/0x124
> | print_address_description+0x60/0x258
> | kasan_report+0x140/0x1a0
> | __asan_report_load8_noabort+0x18/0x20
> | __do_proc_doulongvec_minmax+0x5d8/0x6a0
> | proc_doulongvec_minmax+0x4c/0x78
> | proc_sys_call_handler.isra.19+0x144/0x1d8
> | proc_sys_write+0x34/0x58
> | __vfs_write+0x54/0xe8
> | vfs_write+0x124/0x3c0
> | ksys_write+0xbc/0x168
> | __arm64_sys_write+0x68/0x98
> | el0_svc_common+0x100/0x258
> | el0_svc_handler+0x48/0xc0
> | el0_svc+0x8/0xc
> |
> | The buggy address belongs to the variable:
> | zero+0x0/0x40
> |
> | Memory state around the buggy address:
> | ffff2000133d1b00: 00 00 00 00 00 00 00 00 fa fa fa fa 04 fa fa fa
> | ffff2000133d1b80: fa fa fa fa 04 fa fa fa fa fa fa fa 04 fa fa fa
> | >ffff2000133d1c00: fa fa fa fa 04 fa fa fa fa fa fa fa 00 00 00 00
> | ^
> | ffff2000133d1c80: fa fa fa fa 00 fa fa fa fa fa fa fa 00 00 00 00
> | ffff2000133d1d00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
>
> Fix the splat by introducing a unsigned long 'zero_ul' and using that
> instead.
>
> Fixes: 32a5ad9c2285 ("sysctl: handle overflow for file-max")
> Cc: Christian Brauner <christ...@brauner.io>
> Cc: Kees Cook <keesc...@chromium.org>
> Cc: Andrew Morton <a...@linux-foundation.org>
> Signed-off-by: Will Deacon <will.dea...@arm.com>
Hey Will, thanks! For the record, there's another patch by Matteo (Cced)
for the same thing:
https://lore.kernel.org/lkml/20190328130306.25384-1-mcr...@redhat.com/
He's proposing a slightly different version for the fix. I don't care
very much but I like that you've used the explicit unsigned long so:
Acked-by: Christian Brauner <christ...@brauner.io>
Thanks to both of you for fixing this!
> ---
> kernel/sysctl.c | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/sysctl.c b/kernel/sysctl.c
> index e5da394d1ca3..c9ec050bcf46 100644
> --- a/kernel/sysctl.c
> +++ b/kernel/sysctl.c
> @@ -128,6 +128,7 @@ static int zero;
> static int __maybe_unused one = 1;
> static int __maybe_unused two = 2;
> static int __maybe_unused four = 4;
> +static unsigned long zero_ul;
> static unsigned long one_ul = 1;
> static unsigned long long_max = LONG_MAX;
> static int one_hundred = 100;
> @@ -1750,7 +1751,7 @@ static struct ctl_table fs_table[] = {
> .maxlen = sizeof(files_stat.max_files),
> .mode = 0644,
> .proc_handler = proc_doulongvec_minmax,
> - .extra1 = &zero,
> + .extra1 = &zero_ul,
> .extra2 = &long_max,
> },
> {
> --
> 2.11.0
>
