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([2a0b:e7c0:0:107::aaaa:69]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4266f6f10ebsm109161495e9.16.2024.07.11.01.44.19 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 11 Jul 2024 01:44:20 -0700 (PDT) Message-ID: <4827a532-a54a-483a-9222-692aea33a611@kernel.org> Date: Thu, 11 Jul 2024 10:44:18 +0200 Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4] x86/entry_32: Use stack segment selector for VERW operand To: Uros Bizjak , Pawan Gupta Cc: Borislav Petkov , Dave Hansen , linux-kernel@vger.kernel.org, x86@kernel.org, Robert Gill , Jari Ruusu , Brian Gerst , "Linux regression tracking (Thorsten Leemhuis)" , antonio.gomez.iglesias@linux.intel.com, daniel.sneddon@linux.intel.com, stable@vger.kernel.org References: <20240710-fix-dosemu-vm86-v4-1-aa6464e1de6f@linux.intel.com> <8551ef61-71fb-18f3-a8a8-6c7c3ed731e6@gmail.com> <20240710231609.rbxd7m5mjk53rthl@desk> Content-Language: en-US From: Jiri Slaby Autocrypt: addr=jirislaby@kernel.org; keydata= xsFNBE6S54YBEACzzjLwDUbU5elY4GTg/NdotjA0jyyJtYI86wdKraekbNE0bC4zV+ryvH4j rrcDwGs6tFVrAHvdHeIdI07s1iIx5R/ndcHwt4fvI8CL5PzPmn5J+h0WERR5rFprRh6axhOk rSD5CwQl19fm4AJCS6A9GJtOoiLpWn2/IbogPc71jQVrupZYYx51rAaHZ0D2KYK/uhfc6neJ i0WqPlbtIlIrpvWxckucNu6ZwXjFY0f3qIRg3Vqh5QxPkojGsq9tXVFVLEkSVz6FoqCHrUTx wr+aw6qqQVgvT/McQtsI0S66uIkQjzPUrgAEtWUv76rM4ekqL9stHyvTGw0Fjsualwb0Gwdx ReTZzMgheAyoy/umIOKrSEpWouVoBt5FFSZUyjuDdlPPYyPav+hpI6ggmCTld3u2hyiHji2H cDpcLM2LMhlHBipu80s9anNeZhCANDhbC5E+NZmuwgzHBcan8WC7xsPXPaiZSIm7TKaVoOcL 9tE5aN3jQmIlrT7ZUX52Ff/hSdx/JKDP3YMNtt4B0cH6ejIjtqTd+Ge8sSttsnNM0CQUkXps w98jwz+Lxw/bKMr3NSnnFpUZaxwji3BC9vYyxKMAwNelBCHEgS/OAa3EJoTfuYOK6wT6nadm YqYjwYbZE5V/SwzMbpWu7Jwlvuwyfo5mh7w5iMfnZE+vHFwp/wARAQABzSFKaXJpIFNsYWJ5 IDxqaXJpc2xhYnlAa2VybmVsLm9yZz7CwXcEEwEIACEFAlW3RUwCGwMFCwkIBwIGFQgJCgsC BBYCAwECHgECF4AACgkQvSWxBAa0cEnVTg//TQpdIAr8Tn0VAeUjdVIH9XCFw+cPSU+zMSCH eCZoA/N6gitEcnvHoFVVM7b3hK2HgoFUNbmYC0RdcSc80pOF5gCnACSP9XWHGWzeKCARRcQR 4s5YD8I4VV5hqXcKo2DFAtIOVbHDW+0okOzcecdasCakUTr7s2fXz97uuoc2gIBB7bmHUGAH XQXHvdnCLjDjR+eJN+zrtbqZKYSfj89s/ZHn5Slug6w8qOPT1sVNGG+eWPlc5s7XYhT9z66E l5C0rG35JE4PhC+tl7BaE5IwjJlBMHf/cMJxNHAYoQ1hWQCKOfMDQ6bsEr++kGUCbHkrEFwD UVA72iLnnnlZCMevwE4hc0zVhseWhPc/KMYObU1sDGqaCesRLkE3tiE7X2cikmj/qH0CoMWe gjnwnQ2qVJcaPSzJ4QITvchEQ+tbuVAyvn9H+9MkdT7b7b2OaqYsUP8rn/2k1Td5zknUz7iF oJ0Z9wPTl6tDfF8phaMIPISYrhceVOIoL+rWfaikhBulZTIT5ihieY9nQOw6vhOfWkYvv0Dl o4GRnb2ybPQpfEs7WtetOsUgiUbfljTgILFw3CsPW8JESOGQc0Pv8ieznIighqPPFz9g+zSu Ss/rpcsqag5n9rQp/H3WW5zKUpeYcKGaPDp/vSUovMcjp8USIhzBBrmI7UWAtuedG9prjqfO wU0ETpLnhgEQAM+cDWLL+Wvc9cLhA2OXZ/gMmu7NbYKjfth1UyOuBd5emIO+d4RfFM02XFTI t4MxwhAryhsKQQcA4iQNldkbyeviYrPKWjLTjRXT5cD2lpWzr+Jx7mX7InV5JOz1Qq+P+nJW YIBjUKhI03ux89p58CYil24Zpyn2F5cX7U+inY8lJIBwLPBnc9Z0An/DVnUOD+0wIcYVnZAK DiIXODkGqTg3fhZwbbi+KAhtHPFM2fGw2VTUf62IHzV+eBSnamzPOBc1XsJYKRo3FHNeLuS8 f4wUe7bWb9O66PPFK/RkeqNX6akkFBf9VfrZ1rTEKAyJ2uqf1EI1olYnENk4+00IBa+BavGQ 8UW9dGW3nbPrfuOV5UUvbnsSQwj67pSdrBQqilr5N/5H9z7VCDQ0dhuJNtvDSlTf2iUFBqgk 3smln31PUYiVPrMP0V4ja0i9qtO/TB01rTfTyXTRtqz53qO5dGsYiliJO5aUmh8swVpotgK4 /57h3zGsaXO9PGgnnAdqeKVITaFTLY1ISg+Ptb4KoliiOjrBMmQUSJVtkUXMrCMCeuPDGHo7 39Xc75lcHlGuM3yEB//htKjyprbLeLf1y4xPyTeeF5zg/0ztRZNKZicgEmxyUNBHHnBKHQxz 1j+mzH0HjZZtXjGu2KLJ18G07q0fpz2ZPk2D53Ww39VNI/J9ABEBAAHCwV8EGAECAAkFAk6S 54YCGwwACgkQvSWxBAa0cEk3tRAAgO+DFpbyIa4RlnfpcW17AfnpZi9VR5+zr496n2jH/1ld wRO/S+QNSA8qdABqMb9WI4BNaoANgcg0AS429Mq0taaWKkAjkkGAT7mD1Q5PiLr06Y/+Kzdr 90eUVneqM2TUQQbK+Kh7JwmGVrRGNqQrDk+gRNvKnGwFNeTkTKtJ0P8jYd7P1gZb9Fwj9YLx jhn/sVIhNmEBLBoI7PL+9fbILqJPHgAwW35rpnq4f/EYTykbk1sa13Tav6btJ+4QOgbcezWI wZ5w/JVfEJW9JXp3BFAVzRQ5nVrrLDAJZ8Y5ioWcm99JtSIIxXxt9FJaGc1Bgsi5K/+dyTKL wLMJgiBzbVx8G+fCJJ9YtlNOPWhbKPlrQ8+AY52Aagi9WNhe6XfJdh5g6ptiOILm330mkR4g W6nEgZVyIyTq3ekOuruftWL99qpP5zi+eNrMmLRQx9iecDNgFr342R9bTDlb1TLuRb+/tJ98 f/bIWIr0cqQmqQ33FgRhrG1+Xml6UXyJ2jExmlO8JljuOGeXYh6ZkIEyzqzffzBLXZCujlYQ DFXpyMNVJ2ZwPmX2mWEoYuaBU0JN7wM+/zWgOf2zRwhEuD3A2cO2PxoiIfyUEfB9SSmffaK/ S4xXoB6wvGENZ85Hg37C7WDNdaAt6Xh2uQIly5grkgvWppkNy4ZHxE+jeNsU7tg= In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 11. 07. 24, 10:30, Uros Bizjak wrote: > On Thu, Jul 11, 2024 at 1:16 AM Pawan Gupta > wrote: >> >> On Wed, Jul 10, 2024 at 11:50:50PM +0200, Uros Bizjak wrote: >>> >>> >>> On 10. 07. 24 21:06, Pawan Gupta wrote: >>>> Robert Gill reported below #GP when dosemu software was executing vm86() >>>> system call: >>>> >>>> general protection fault: 0000 [#1] PREEMPT SMP >>>> CPU: 4 PID: 4610 Comm: dosemu.bin Not tainted 6.6.21-gentoo-x86 #1 >>>> Hardware name: Dell Inc. PowerEdge 1950/0H723K, BIOS 2.7.0 10/30/2010 >>>> EIP: restore_all_switch_stack+0xbe/0xcf >>>> EAX: 00000000 EBX: 00000000 ECX: 00000000 EDX: 00000000 >>>> ESI: 00000000 EDI: 00000000 EBP: 00000000 ESP: ff8affdc >>>> DS: 0000 ES: 0000 FS: 0000 GS: 0033 SS: 0068 EFLAGS: 00010046 >>>> CR0: 80050033 CR2: 00c2101c CR3: 04b6d000 CR4: 000406d0 >>>> Call Trace: >>>> show_regs+0x70/0x78 >>>> die_addr+0x29/0x70 >>>> exc_general_protection+0x13c/0x348 >>>> exc_bounds+0x98/0x98 >>>> handle_exception+0x14d/0x14d >>>> exc_bounds+0x98/0x98 >>>> restore_all_switch_stack+0xbe/0xcf >>>> exc_bounds+0x98/0x98 >>>> restore_all_switch_stack+0xbe/0xcf >>>> >>>> This only happens when VERW based mitigations like MDS/RFDS are enabled. >>>> This is because segment registers with an arbitrary user value can result >>>> in #GP when executing VERW. Intel SDM vol. 2C documents the following >>>> behavior for VERW instruction: >>>> >>>> #GP(0) - If a memory operand effective address is outside the CS, DS, ES, >>>> FS, or GS segment limit. >>>> >>>> CLEAR_CPU_BUFFERS macro executes VERW instruction before returning to user >>>> space. Replace CLEAR_CPU_BUFFERS with a safer version that uses %ss to >>>> refer VERW operand mds_verw_sel. This ensures VERW will not #GP for an >>>> arbitrary user %ds. Also, in NMI return path, move VERW to after >>>> RESTORE_ALL_NMI that touches GPRs. >>>> >>>> For clarity, below are the locations where the new CLEAR_CPU_BUFFERS_SAFE >>>> version is being used: >>>> >>>> * entry_INT80_32(), entry_SYSENTER_32() and interrupts (via >>>> handle_exception_return) do: >>>> >>>> restore_all_switch_stack: >>>> [...] >>>> mov %esi,%esi >>>> verw %ss:0xc0fc92c0 <------------- >>>> iret >>>> >>>> * Opportunistic SYSEXIT: >>>> >>>> [...] >>>> verw %ss:0xc0fc92c0 <------------- >>>> btrl $0x9,(%esp) >>>> popf >>>> pop %eax >>>> sti >>>> sysexit >>>> >>>> * nmi_return and nmi_from_espfix: >>>> mov %esi,%esi >>>> verw %ss:0xc0fc92c0 <------------- >>>> jmp .Lirq_return >>>> >>>> Fixes: a0e2dab44d22 ("x86/entry_32: Add VERW just before userspace transition") >>>> Cc: stable@vger.kernel.org # 5.10+ >>>> Reported-by: Robert Gill >>>> Closes: https://bugzilla.kernel.org/show_bug.cgi?id=218707 >>>> Closes: https://lore.kernel.org/all/8c77ccfd-d561-45a1-8ed5-6b75212c7a58@leemhuis.info/ >>>> Suggested-by: Dave Hansen >>>> Suggested-by: Brian Gerst # Use %ss >>>> Signed-off-by: Pawan Gupta >>>> --- >>>> v4: >>>> - Further simplify the patch by using %ss for all VERW calls in 32-bit mode (Brian). >>>> - In NMI exit path move VERW after RESTORE_ALL_NMI that touches GPRs (Dave). >>>> >>>> v3: https://lore.kernel.org/r/20240701-fix-dosemu-vm86-v3-1-b1969532c75a@linux.intel.com >>>> - Simplify CLEAR_CPU_BUFFERS_SAFE by using %ss instead of %ds (Brian). >>>> - Do verw before popf in SYSEXIT path (Jari). >>>> >>>> v2: https://lore.kernel.org/r/20240627-fix-dosemu-vm86-v2-1-d5579f698e77@linux.intel.com >>>> - Safe guard against any other system calls like vm86() that might change %ds (Dave). >>>> >>>> v1: https://lore.kernel.org/r/20240426-fix-dosemu-vm86-v1-1-88c826a3f378@linux.intel.com >>>> --- >>>> >>>> --- >>>> arch/x86/entry/entry_32.S | 18 +++++++++++++++--- >>>> 1 file changed, 15 insertions(+), 3 deletions(-) >>>> >>>> diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S >>>> index d3a814efbff6..d54f6002e5a0 100644 >>>> --- a/arch/x86/entry/entry_32.S >>>> +++ b/arch/x86/entry/entry_32.S >>>> @@ -253,6 +253,16 @@ >>>> .Lend_\@: >>>> .endm >>>> +/* >>>> + * Safer version of CLEAR_CPU_BUFFERS that uses %ss to reference VERW operand >>>> + * mds_verw_sel. This ensures VERW will not #GP for an arbitrary user %ds. >>>> + */ >>>> +.macro CLEAR_CPU_BUFFERS_SAFE >>>> + ALTERNATIVE "jmp .Lskip_verw\@", "", X86_FEATURE_CLEAR_CPU_BUF >>>> + verw %ss:_ASM_RIP(mds_verw_sel) >>>> +.Lskip_verw\@: >>>> +.endm >>> >>> Why not simply: >>> >>> .macro CLEAR_CPU_BUFFERS_SAFE >>> ALTERNATIVE "", __stringify(verw %ss:_ASM_RIP(mds_verw_sel)), >>> X86_FEATURE_CLEAR_CPU_BUF >>> .endm >> >> We can do it this way as well. But, there are stable kernels that don't >> support relocations in ALTERNATIVEs. The way it is done in current patch >> can be backported without worrying about which kernels support relocations. > > Then you could use absolute reference in backports to kernels that > don't support relocations in ALTERNATIVEs, "verw %ss:mds_verw_sel" > works as well, but the relocation is one byte larger. No, the kernels support (and use) relocations. It's only ALTERNATIVEs in older kernels don't. -- js suse labs