Patch "KVM: arm64: Fix kernel BUG() due to bad backport of FPSIMD/SVE/SME fix" has been added to the 5.15-stable tree

[<gregkh@linuxfoundation.org>]

This is a note to let you know that I've just added the patch titled

    KVM: arm64: Fix kernel BUG() due to bad backport of FPSIMD/SVE/SME fix

to the 5.15-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     kvm-arm64-fix-kernel-bug-due-to-bad-backport-of-fpsimd-sve-sme-fix.patch
and it can be found in the queue-5.15 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@vger.kernel.org> know about it.


From will@kernel.org  Fri Aug 22 16:24:21 2025
From: Will Deacon <will@kernel.org>
Date: Fri, 22 Aug 2025 15:04:02 +0100
Subject: KVM: arm64: Fix kernel BUG() due to bad backport of FPSIMD/SVE/SME fix
To: stable@vger.kernel.org
Cc: linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev, Will Deacon <will@kernel.org>, Ard Biesheuvel <ardb@kernel.org>, Lee Jones <lee@kernel.org>, Sasha Levin <sashal@kernel.org>, Greg Kroah-Hartman <gregkh@linuxfoundation.org>, Mark Rutland <mark.rutland@arm.com>, Fuad Tabba <tabba@google.com>, Marc Zyngier <maz@kernel.org>
Message-ID: <20250822140402.2688-1-will@kernel.org>

From: Will Deacon <will@kernel.org>

Upstream commit fbc7e61195e2 ("KVM: arm64: Unconditionally save+flush
host FPSIMD/SVE/SME state") relies on interrupts being disabled during
fpsimd_save_and_flush_cpu_state() so that a softirq cannot be taken
while the host floating point context is being saved and potentially try
to use kernel-mode NEON.

Unfortunately, stable kernels without 9b19700e623f ("arm64: fpsimd: Drop
unneeded 'busy' flag") leave interrupts enabled in
fpsimd_save_and_flush_cpu_state() and so the BUG_ON(!may_use_simd()) in
kernel_neon_begin() has been observed to trigger in real-world usage:

 |  kernel BUG at arch/arm64/kernel/fpsimd.c:1904!
 |  Internal error: Oops - BUG: 00000000f2000800 [#1] PREEMPT SMP
 |
 |  Call trace:
 |   kernel_neon_begin+0xdc/0x12c
 |   ...
 |   crypto_aead_decrypt+0x5c/0x6c
 |   seqiv_aead_decrypt+0x88/0x9c
 |   crypto_aead_decrypt+0x5c/0x6c
 |   esp_input+0x280/0x364
 |   xfrm_input+0x6ac/0x16f8
 |   ...
 |   net_rx_action+0x13c/0x31c
 |   handle_softirqs+0x124/0x3d0
 |   __do_softirq+0x14/0x20
 |   ____do_softirq+0x10/0x20
 |   call_on_irq_stack+0x3c/0x74
 |   do_softirq_own_stack+0x1c/0x2c
 |   __irq_exit_rcu+0x54/0xb4
 |   irq_exit_rcu+0x10/0x1c
 |   el1_interrupt+0x38/0x58
 |   el1h_64_irq_handler+0x18/0x24
 |   el1h_64_irq+0x68/0x6c
 |   fpsimd_save+0xe4/0x130
 |   kvm_arch_vcpu_load_fp+0x2c/0x58
 |   kvm_arch_vcpu_load+0x88/0x26c
 |   kvm_sched_in+0x2c/0x3c

Given that 9b19700e623f ("arm64: fpsimd: Drop unneeded 'busy' flag") is
not a fix in its own right, has non-trivial dependencies and is a
reasonably invasive change to the in-kernel use of fpsimd, opt instead
for a simple fix to use the softirq-safe {get,put}_cpu_fpsimd_context()
helpers in fpsimd_save_and_flush_cpu_state().

Cc: Ard Biesheuvel <ardb@kernel.org>
Cc: Lee Jones <lee@kernel.org>
Cc: Sasha Levin <sashal@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Fuad Tabba <tabba@google.com>
Cc: Marc Zyngier <maz@kernel.org>
Cc: <stable@vger.kernel.org> # 5.15.y, 6.1.y and 6.6.y
Fixes: 806d5c1e1d2e ("KVM: arm64: Unconditionally save+flush host FPSIMD/SVE/SME state") # 6.6.y
Fixes: 04c50cc23a49 ("KVM: arm64: Unconditionally save+flush host FPSIMD/SVE/SME state") # 6.1.y
Fixes: 5289ac43b69c ("KVM: arm64: Unconditionally save+flush host FPSIMD/SVE/SME state") # 5.15.y
Signed-off-by: Will Deacon <will@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 arch/arm64/kernel/fpsimd.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

--- a/arch/arm64/kernel/fpsimd.c
+++ b/arch/arm64/kernel/fpsimd.c
@@ -1303,10 +1303,10 @@ void fpsimd_save_and_flush_cpu_state(voi
 	if (!system_supports_fpsimd())
 		return;
 	WARN_ON(preemptible());
-	__get_cpu_fpsimd_context();
+	get_cpu_fpsimd_context();
 	fpsimd_save();
 	fpsimd_flush_cpu_state();
-	__put_cpu_fpsimd_context();
+	put_cpu_fpsimd_context();
 }
 
 #ifdef CONFIG_KERNEL_MODE_NEON


Patches currently in stable-queue which might be from will@kernel.org are

queue-5.15/kvm-arm64-fix-kernel-bug-due-to-bad-backport-of-fpsimd-sve-sme-fix.patch
queue-5.15/arm64-mark-kernel-as-tainted-on-sae-and-serror-panic.patch
queue-5.15/arm64-entry-mask-daif-in-cpu_switch_to-call_on_irq_stack.patch
queue-5.15/mm-ptdump-take-the-memory-hotplug-lock-inside-ptdump_walk_pgd.patch