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SubjectRe: [PATCH -next] x86/mm/pat: mark an intentional data race
On Wed, Mar 11, 2020 at 05:17:56PM +0100, Borislav Petkov wrote:
> + Paul.
>
> On Mon, Feb 10, 2020 at 09:10:16AM -0500, Qian Cai wrote:
> > cpa_4k_install could be accessed concurrently as noticed by KCSAN,
> >
> > read to 0xffffffffaa59a000 of 8 bytes by interrupt on cpu 7:
> > cpa_inc_4k_install arch/x86/mm/pat/set_memory.c:131 [inline]
> > __change_page_attr+0x10cf/0x1840 arch/x86/mm/pat/set_memory.c:1514
> > __change_page_attr_set_clr+0xce/0x490 arch/x86/mm/pat/set_memory.c:1636
> > __set_pages_np+0xc4/0xf0 arch/x86/mm/pat/set_memory.c:2148
> > __kernel_map_pages+0xb0/0xc8 arch/x86/mm/pat/set_memory.c:2178
> > kernel_map_pages include/linux/mm.h:2719 [inline] <snip>
> >
> > write to 0xffffffffaa59a000 of 8 bytes by task 1 on cpu 6:
> > cpa_inc_4k_install arch/x86/mm/pat/set_memory.c:131 [inline]
> > __change_page_attr+0x10ea/0x1840 arch/x86/mm/pat/set_memory.c:1514
> > __change_page_attr_set_clr+0xce/0x490 arch/x86/mm/pat/set_memory.c:1636
> > __set_pages_p+0xc4/0xf0 arch/x86/mm/pat/set_memory.c:2129
> > __kernel_map_pages+0x2e/0xc8 arch/x86/mm/pat/set_memory.c:2176
> > kernel_map_pages include/linux/mm.h:2719 [inline] <snip>
> >
> > Both accesses are due to the same "cpa_4k_install++" in
> > cpa_inc_4k_install. A data race here could be potentially undesirable:
> > depending on compiler optimizations or how x86 executes a non-LOCK'd
> > increment, it may lose increments, corrupt the counter, etc. Since this
> > counter only seems to be used for printing some stats, this data race
> > itself is unlikely to cause harm to the system though. Thus, mark this
> > intentional data race using the data_race() marco.
> >
> > Suggested-by: Macro Elver <elver@google.com>
> > Signed-off-by: Qian Cai <cai@lca.pw>
> > ---
> > arch/x86/mm/pat/set_memory.c | 2 +-
> > 1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/arch/x86/mm/pat/set_memory.c b/arch/x86/mm/pat/set_memory.c
> > index c4aedd00c1ba..ea0b6df950ee 100644
> > --- a/arch/x86/mm/pat/set_memory.c
> > +++ b/arch/x86/mm/pat/set_memory.c
> > @@ -128,7 +128,7 @@ static inline void cpa_inc_2m_checked(void)
> >
> > static inline void cpa_inc_4k_install(void)
> > {
> > - cpa_4k_install++;
> > + data_race(cpa_4k_install++);
> > }
> >
> > static inline void cpa_inc_lp_sameprot(int level)
> > --
>
> Acked-by: Borislav Petkov <bp@suse.de>

Applied, thank you both!

Thanx, Paul

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