[PATCH] x86-32: fix tlb flushing when lguest clears PGE

[Daniel Borkmann]

Fengguang reported [1] random corruptions from various locations on
x86-32 after commits d2852a224050 ("arch: add ARCH_HAS_SET_MEMORY
config") and 9d876e79df6a ("bpf: fix unlocking of jited image when
module ronx not set") that uses the former. While x86-32 doesn't
have a JIT like x86_64, the bpf_prog_lock_ro() and bpf_prog_unlock_ro()
got enabled due to ARCH_HAS_SET_MEMORY, whereas Fengguang's test
kernel doesn't have module support built in and therefore never
had the DEBUG_SET_MODULE_RONX setting enabled.

After investigating the crashes further, it turned out that using
set_memory_ro() and set_memory_rw() didn't have the desired effect,
for example, setting the pages as read-only on x86-32 would still
let probe_kernel_write() succeed without error. This behavior would
manifest itself in situations where the vmalloc'ed buffer was accessed
prior to set_memory_*() such as in case of bpf_prog_alloc(). In
cases where it wasn't, the page attribute changes seemed to have
taken effect, leading to the conclusion that a TLB invalidate
didn't happen. Moreover, it turned out that this issue reproduced
with qemu in "-cpu kvm64" mode, but not for "-cpu host". When the
issue occurs, change_page_attr_set_clr() did trigger a TLB flush
as expected via __flush_tlb_all() through cpa_flush_range(), though.

There are 3 variants for issuing a TLB flush: invpcid_flush_all()
(depends on CPU feature bits X86_FEATURE_INVPCID, X86_FEATURE_PGE),
cr4 based flush (depends on X86_FEATURE_PGE), and cr3 based flush.
For "-cpu host" case in my setup, the flush used invpcid_flush_all()
variant, whereas for "-cpu kvm64", the flush was cr4 based. Switching
the kvm64 case to cr3 manually worked fine, and further investigating
the cr4 one turned out that X86_CR4_PGE bit was not set in cr4
register, meaning the __native_flush_tlb_global_irq_disabled() wrote
cr4 twice with the same value instead of clearing X86_CR4_PGE in the
first write to trigger the flush.

It turned out that X86_CR4_PGE was cleared from cr4 during init
from lguest_arch_host_init() via adjust_pge(). The X86_FEATURE_PGE
bit is also cleared from there due to concerns of using PGE in
guest kernel that can lead to hard to trace bugs (see bff672e630a0
("lguest: documentation V: Host") in init()). The CPU feature bits
are cleared in dynamic boot_cpu_data, but they never propagated to
__flush_tlb_all() as it uses static_cpu_has() instead of boot_cpu_has()
for testing which variant of TLB flushing to use, meaning they still
used the old setting of the host kernel.

Clearing via setup_clear_cpu_cap(X86_FEATURE_PGE) so this would
propagate to static_cpu_has() checks is too late at this point as
sections have been patched already, so for now, it seems reasonable
to switch back to boot_cpu_has(X86_FEATURE_PGE) as it was prior to
commit c109bf95992b ("x86/cpufeature: Remove cpu_has_pge"). This
lets the TLB flush trigger via cr3 as originally intended, properly
makes the new page attributes visible and thus fixes the crashes
seen by Fengguang.

  [1] https://lkml.org/lkml/2017/3/1/344

Fixes: c109bf95992b ("x86/cpufeature: Remove cpu_has_pge")
Reported-by: Fengguang Wu <fengguang.wu@xxxxxxxxx>
Signed-off-by: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Kees Cook <keescook@xxxxxxxxxxxx>
Cc: Laura Abbott <labbott@xxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: H. Peter Anvin <hpa@xxxxxxxxx>
Cc: Rusty Russell <rusty@xxxxxxxxxxxxxxx>
Cc: Alexei Starovoitov <ast@xxxxxxxxxx>
Cc: David S. Miller <davem@xxxxxxxxxxxxx>
---
 arch/x86/include/asm/tlbflush.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/tlbflush.h b/arch/x86/include/asm/tlbflush.h
index 6fa8594..fc5abff 100644
--- a/arch/x86/include/asm/tlbflush.h
+++ b/arch/x86/include/asm/tlbflush.h
@@ -188,7 +188,7 @@ static inline void __native_flush_tlb_single(unsigned long addr)
 
 static inline void __flush_tlb_all(void)
 {
-	if (static_cpu_has(X86_FEATURE_PGE))
+	if (boot_cpu_has(X86_FEATURE_PGE))
 		__flush_tlb_global();
 	else
 		__flush_tlb();
-- 
1.9.3