On Thu, Apr 4, 2019 at 11:48 PM Hou Tao <houtao1@xxxxxxxxxx> wrote: > > We have got KASAN splat when tried to set /proc/sys/fs/file-max: > > BUG: KASAN: global-out-of-bounds in __do_proc_doulongvec_minmax+0x3e4/0x8f0 > Read of size 8 at addr ffff20000f9b2980 by task file-max.sh/36819 > > Call trace: > dump_backtrace+0x0/0x3f8 > show_stack+0x3c/0x60 > dump_stack+0x150/0x1a8 > print_address_description+0x2b8/0x5a0 > kasan_report+0x278/0x648 > __asan_load8+0x124/0x170 > __do_proc_doulongvec_minmax+0x3e4/0x8f0 > proc_doulongvec_minmax+0x80/0xa0 > proc_sys_call_handler+0x188/0x2a0 > proc_sys_write+0x5c/0x80 > __vfs_write+0x118/0x578 > vfs_write+0x184/0x418 > ksys_write+0xfc/0x1e8 > __arm64_sys_write+0x88/0xa8 > el0_svc_common+0x1a4/0x500 > el0_svc_handler+0xb8/0x180 > el0_svc+0x8/0xc > > The buggy address belongs to the variable: > zero+0x0/0x40 > > The cause is that proc_doulongvec_minmax() is trying to cast an int > pointer (namely &zero) to a unsigned long pointer, and dereferencing it. > > Although the warning seems does no harm, because zero will be placed > in a .bss section, but it's better to kill the KASAN warning by > redefining zero as a unsigned long, so it's OK whenever it is accessed > as an int or a a unsigned long. > > An alternative fix seems to be set the minimal value of file-max to be 1, > so one_ul can be used instead, but I'm not sure whether or not a file-max > with a value of zero has special purpose (e.g., prohibit the file-related > activities of all no-privileged users). > > Signed-off-by: Hou Tao <houtao1@xxxxxxxxxx> Acked-by: Kees Cook <keescook@xxxxxxxxxxxx> -Kees > --- > kernel/sysctl.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/kernel/sysctl.c b/kernel/sysctl.c > index e5da394d1ca3..03846e015013 100644 > --- a/kernel/sysctl.c > +++ b/kernel/sysctl.c > @@ -124,7 +124,7 @@ static int sixty = 60; > > static int __maybe_unused neg_one = -1; > > -static int zero; > +static unsigned long zero; > static int __maybe_unused one = 1; > static int __maybe_unused two = 2; > static int __maybe_unused four = 4; > -- > 2.16.2.dirty > -- Kees Cook
