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Date: Tue, 18 Jun 2024 14:41:22 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Sean Christopherson <seanjc@...gle.com>, pbonzini@...hat.com, Babu Moger <babu.moger@....com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] KVM: Fix Undefined Behavior Sanitizer(UBSAN) error
On Wed, 12 Jun 2024 09:41:51 -0500, Babu Moger wrote:
> System throws this following UBSAN: invalid-load error when the very first
> VM is powered up on a freshly booted host machine. Happens only with 2P or
> 4P (multiple sockets) systems.
>
> { 688.429145] ------------[ cut here ]------------
> [ 688.429156] UBSAN: invalid-load in arch/x86/kvm/../../../virt/kvm/kvm_main.c:655:10
> [ 688.437739] load of value 160 is not a valid value for type '_Bool'
> [ 688.444760] CPU: 370 PID: 8246 Comm: CPU 0/KVM Not tainted 6.8.2-amdsos-build58-ubuntu-22.04+ #1
> [ 688.444767] Hardware name: AMD Corporation Sh54p/Sh54p, BIOS WPC4429N 04/25/2024
> [ 688.444770] Call Trace:
> [ 688.444777] <TASK>
> [ 688.444787] dump_stack_lvl+0x48/0x60
> [ 688.444810] ubsan_epilogue+0x5/0x30
> [ 688.444823] __ubsan_handle_load_invalid_value+0x79/0x80
> [ 688.444827] ? srso_alias_return_thunk+0x5/0xfbef5
> [ 688.444836] ? flush_tlb_func+0xe9/0x2e0
> [ 688.444845] kvm_mmu_notifier_invalidate_range_end.cold+0x18/0x4f [kvm]
> [ 688.444906] __mmu_notifier_invalidate_range_end+0x63/0xe0
> [ 688.444917] __split_huge_pmd+0x367/0xfc0
> [ 688.444928] ? srso_alias_return_thunk+0x5/0xfbef5
> [ 688.444931] ? alloc_pages_mpol+0x97/0x210
> [ 688.444941] do_huge_pmd_wp_page+0x1cc/0x380
> [ 688.444946] __handle_mm_fault+0x8ee/0xe50
> [ 688.444958] handle_mm_fault+0xe4/0x4a0
> [ 688.444962] __get_user_pages+0x190/0x840
> [ 688.444972] get_user_pages_unlocked+0xe0/0x590
> [ 688.444977] hva_to_pfn+0x114/0x550 [kvm]
> [ 688.445007] ? srso_alias_return_thunk+0x5/0xfbef5
> [ 688.445011] ? __gfn_to_pfn_memslot+0x3b/0xd0 [kvm]
> [ 688.445037] kvm_faultin_pfn+0xed/0x5b0 [kvm]
> [ 688.445079] kvm_tdp_page_fault+0x123/0x170 [kvm]
> [ 688.445109] kvm_mmu_page_fault+0x244/0xaa0 [kvm]
> [ 688.445136] ? srso_alias_return_thunk+0x5/0xfbef5
> [ 688.445138] ? kvm_io_bus_get_first_dev+0x56/0xf0 [kvm]
> [ 688.445165] ? srso_alias_return_thunk+0x5/0xfbef5
> [ 688.445171] ? svm_vcpu_run+0x329/0x7c0 [kvm_amd]
> [ 688.445186] vcpu_enter_guest+0x592/0x1070 [kvm]
> [ 688.445223] kvm_arch_vcpu_ioctl_run+0x145/0x8a0 [kvm]
> [ 688.445254] kvm_vcpu_ioctl+0x288/0x6d0 [kvm]
> [ 688.445279] ? vcpu_put+0x22/0x50 [kvm]
> [ 688.445305] ? srso_alias_return_thunk+0x5/0xfbef5
> [ 688.445307] ? kvm_arch_vcpu_ioctl_run+0x346/0x8a0 [kvm]
> [ 688.445335] __x64_sys_ioctl+0x8f/0xd0
> [ 688.445343] do_syscall_64+0x77/0x120
> [ 688.445353] ? srso_alias_return_thunk+0x5/0xfbef5
> [ 688.445355] ? fire_user_return_notifiers+0x42/0x70
> [ 688.445363] ? srso_alias_return_thunk+0x5/0xfbef5
> [ 688.445365] ? syscall_exit_to_user_mode+0x82/0x1b0
> [ 688.445372] ? srso_alias_return_thunk+0x5/0xfbef5
> [ 688.445377] ? do_syscall_64+0x86/0x120
> [ 688.445380] ? srso_alias_return_thunk+0x5/0xfbef5
> [ 688.445383] ? do_syscall_64+0x86/0x120
> [ 688.445388] ? srso_alias_return_thunk+0x5/0xfbef5
> [ 688.445392] ? do_syscall_64+0x86/0x120
> [ 688.445396] ? srso_alias_return_thunk+0x5/0xfbef5
> [ 688.445400] ? do_syscall_64+0x86/0x120
> [ 688.445404] ? do_syscall_64+0x86/0x120
> [ 688.445407] ? do_syscall_64+0x86/0x120
> [ 688.445410] entry_SYSCALL_64_after_hwframe+0x6e/0x76
> [ 688.445416] RIP: 0033:0x7fdf2ed1a94f
> [ 688.445421] Code: 00 48 89 44 24 18 31 c0 48 8d 44 24 60 c7 04 24 10 00 00 00 48 89
> 44 24 08 48 8d 44 24 20 48 89 44 24 10 b8 10 00 00 00 0f 05 <41>
> 89 c0 3d 00 f0 ff ff 77 1f 48 8b 44 24 18 64 48 2b 04 25 28 00
> [ 688.445424] RSP: 002b:00007fc127bff460 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
> [ 688.445429] RAX: ffffffffffffffda RBX: 000000000000ae80 RCX: 00007fdf2ed1a94f
> [ 688.445432] RDX: 0000000000000000 RSI: 000000000000ae80 RDI: 0000000000000016
> [ 688.445434] RBP: 00005586f80dc350 R08: 00005586f6a0af10 R09: 00000000ffffffff
> [ 688.445436] R10: 0000000000000001 R11: 0000000000000246 R12: 0000000000000000
> [ 688.445438] R13: 0000000000000001 R14: 0000000000000cf8 R15: 0000000000000000
> [ 688.445443] </TASK>
> [ 688.445444] ---[ end trace ]---
Please trim splats like this, the timestamps are uninteresting, and in most cases,
the "? ..." lines are completely superfluous.
The changelog+shortlog should also focus on what the actual bug+fix is. The fact
that UBSAN found the issue is definitely noteworthy, but it's secondary to describing
the actual bug.
Lastly, it's definitely worth calling out that the bug is benign in practice.
kvm_mmu_notifier_invalidate_range_end() is the only caller that passes a null
handler, and it has flush_on_ret=false, i.e. won't actually consume whatever
garbage ends up in r.ret. Not to mention that the compiler elides the entire
sequence on every build I've looked at, even an UBSAN=y builds, i.e. it likely
takes a sanitizer-heavy kernel to actually generate code that yields undefined
behavior at runtime.
KVM: Stop processing *all* memslots when "null" mmu_notifier handler is found
Bail from outer address space loop, not just the inner memslot loop, when
a "null" handler is encountered by __kvm_handle_hva_range(), which is the
intended behavior. On x86, which has multiple address spaces thanks to
SMM emulation, breaking from just the memslot loop results in undefined
behavior due to assigning the non-existent return value from kvm_null_fn()
to a bool.
In practice, the bug is benign as kvm_mmu_notifier_invalidate_range_end()
is the only caller that passes handler=kvm_null_fn, and it doesn't set
flush_on_ret, i.e. assigning garbage to r.ret is ultimately ignored. And
for most configuration the compiler elides the entire sequence, i.e. there
is no undefined behavior at runtime.
------------[ cut here ]------------
UBSAN: invalid-load in arch/x86/kvm/../../../virt/kvm/kvm_main.c:655:10
load of value 160 is not a valid value for type '_Bool'
CPU: 370 PID: 8246 Comm: CPU 0/KVM Not tainted 6.8.2-amdsos-build58-ubuntu-22.04+ #1
Hardware name: AMD Corporation Sh54p/Sh54p, BIOS WPC4429N 04/25/2024
Call Trace:
<TASK>
dump_stack_lvl+0x48/0x60
ubsan_epilogue+0x5/0x30
__ubsan_handle_load_invalid_value+0x79/0x80
kvm_mmu_notifier_invalidate_range_end.cold+0x18/0x4f [kvm]
__mmu_notifier_invalidate_range_end+0x63/0xe0
__split_huge_pmd+0x367/0xfc0
do_huge_pmd_wp_page+0x1cc/0x380
__handle_mm_fault+0x8ee/0xe50
handle_mm_fault+0xe4/0x4a0
__get_user_pages+0x190/0x840
get_user_pages_unlocked+0xe0/0x590
hva_to_pfn+0x114/0x550 [kvm]
kvm_faultin_pfn+0xed/0x5b0 [kvm]
kvm_tdp_page_fault+0x123/0x170 [kvm]
kvm_mmu_page_fault+0x244/0xaa0 [kvm]
vcpu_enter_guest+0x592/0x1070 [kvm]
kvm_arch_vcpu_ioctl_run+0x145/0x8a0 [kvm]
kvm_vcpu_ioctl+0x288/0x6d0 [kvm]
__x64_sys_ioctl+0x8f/0xd0
do_syscall_64+0x77/0x120
entry_SYSCALL_64_after_hwframe+0x6e/0x76
</TASK>
---[ end trace ]---
Fixes: 071064f14d87 ("KVM: Don't take mmu_lock for range invalidation unless necessary")
Signed-off-by: Babu Moger <babu.moger@....com>
Link: https://lore.kernel.org/r/b8723d39903b64c241c50f5513f804390c7b5eec.1718203311.git.babu.moger@amd.com
[sean: massage changelog]
Signed-off-by: Sean Christopherson <seanjc@...gle.com>
Applied to kvm-x86 fixes, with the above shortlog+changelog (I am planning on
send a pull request to Paolo for a few 6.10 fixes on Thursday). Thanks!
[1/1] KVM: Stop processing *all* memslots when "null" mmu_notifier handler is found
https://github.com/kvm-x86/linux/commit/201de7ec31e5
--
https://github.com/kvm-x86/linux/tree/next
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