Re: Patch "arm64: Restrict CPU_BIG_ENDIAN to GNU as or LLVM IAS 15.x or newer" has been added to the 4.19-stable tree

[Sasha Levin <sashal@kernel.org>]

On Mon, Dec 04, 2023 at 09:22:28AM -0700, Nathan Chancellor wrote:
>On Sun, Dec 03, 2023 at 03:23:53PM -0500, Sasha Levin wrote:
>> This is a note to let you know that I've just added the patch titled
>>
>>     arm64: Restrict CPU_BIG_ENDIAN to GNU as or LLVM IAS 15.x or newer
>>
>> to the 4.19-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:
>>      arm64-restrict-cpu_big_endian-to-gnu-as-or-llvm-ias-.patch
>> and it can be found in the queue-4.19 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.
>>
>>
>>
>> commit 4c762ee5a651ad98e47860959d05a8cb18248bdf
>> Author: Nathan Chancellor <nathan@kernel.org>
>> Date:   Wed Oct 25 10:21:28 2023 -0700
>>
>>     arm64: Restrict CPU_BIG_ENDIAN to GNU as or LLVM IAS 15.x or newer
>>
>>     [ Upstream commit 146a15b873353f8ac28dc281c139ff611a3c4848 ]
>>
>>     Prior to LLVM 15.0.0, LLVM's integrated assembler would incorrectly
>>     byte-swap NOP when compiling for big-endian, and the resulting series of
>>     bytes happened to match the encoding of FNMADD S21, S30, S0, S0.
>>
>>     This went unnoticed until commit:
>>
>>       34f66c4c4d5518c1 ("arm64: Use a positive cpucap for FP/SIMD")
>>
>>     Prior to that commit, the kernel would always enable the use of FPSIMD
>>     early in boot when __cpu_setup() initialized CPACR_EL1, and so usage of
>>     FNMADD within the kernel was not detected, but could result in the
>>     corruption of user or kernel FPSIMD state.
>>
>>     After that commit, the instructions happen to trap during boot prior to
>>     FPSIMD being detected and enabled, e.g.
>>
>>     | Unhandled 64-bit el1h sync exception on CPU0, ESR 0x000000001fe00000 -- ASIMD
>>     | CPU: 0 PID: 0 Comm: swapper Not tainted 6.6.0-rc3-00013-g34f66c4c4d55 #1
>>     | Hardware name: linux,dummy-virt (DT)
>>     | pstate: 400000c9 (nZcv daIF -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
>>     | pc : __pi_strcmp+0x1c/0x150
>>     | lr : populate_properties+0xe4/0x254
>>     | sp : ffffd014173d3ad0
>>     | x29: ffffd014173d3af0 x28: fffffbfffddffcb8 x27: 0000000000000000
>>     | x26: 0000000000000058 x25: fffffbfffddfe054 x24: 0000000000000008
>>     | x23: fffffbfffddfe000 x22: fffffbfffddfe000 x21: fffffbfffddfe044
>>     | x20: ffffd014173d3b70 x19: 0000000000000001 x18: 0000000000000005
>>     | x17: 0000000000000010 x16: 0000000000000000 x15: 00000000413e7000
>>     | x14: 0000000000000000 x13: 0000000000001bcc x12: 0000000000000000
>>     | x11: 00000000d00dfeed x10: ffffd414193f2cd0 x9 : 0000000000000000
>>     | x8 : 0101010101010101 x7 : ffffffffffffffc0 x6 : 0000000000000000
>>     | x5 : 0000000000000000 x4 : 0101010101010101 x3 : 000000000000002a
>>     | x2 : 0000000000000001 x1 : ffffd014171f2988 x0 : fffffbfffddffcb8
>>     | Kernel panic - not syncing: Unhandled exception
>>     | CPU: 0 PID: 0 Comm: swapper Not tainted 6.6.0-rc3-00013-g34f66c4c4d55 #1
>>     | Hardware name: linux,dummy-virt (DT)
>>     | Call trace:
>>     |  dump_backtrace+0xec/0x108
>>     |  show_stack+0x18/0x2c
>>     |  dump_stack_lvl+0x50/0x68
>>     |  dump_stack+0x18/0x24
>>     |  panic+0x13c/0x340
>>     |  el1t_64_irq_handler+0x0/0x1c
>>     |  el1_abort+0x0/0x5c
>>     |  el1h_64_sync+0x64/0x68
>>     |  __pi_strcmp+0x1c/0x150
>>     |  unflatten_dt_nodes+0x1e8/0x2d8
>>     |  __unflatten_device_tree+0x5c/0x15c
>>     |  unflatten_device_tree+0x38/0x50
>>     |  setup_arch+0x164/0x1e0
>>     |  start_kernel+0x64/0x38c
>>     |  __primary_switched+0xbc/0xc4
>>
>>     Restrict CONFIG_CPU_BIG_ENDIAN to a known good assembler, which is
>>     either GNU as or LLVM's IAS 15.0.0 and newer, which contains the linked
>>     commit.
>>
>>     Closes: https://github.com/ClangBuiltLinux/linux/issues/1948
>>     Link: https://github.com/llvm/llvm-project/commit/1379b150991f70a5782e9a143c2ba5308da1161c
>>     Signed-off-by: Nathan Chancellor <nathan@kernel.org>
>>     Cc: stable@vger.kernel.org
>>     Acked-by: Mark Rutland <mark.rutland@arm.com>
>>     Link: https://lore.kernel.org/r/20231025-disable-arm64-be-ias-b4-llvm-15-v1-1-b25263ed8b23@kernel.org
>>     Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
>>     Signed-off-by: Sasha Levin <sashal@kernel.org>
>>
>> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
>> index 92646f12a66d2..ff8e0db1693e9 100644
>> --- a/arch/arm64/Kconfig
>> +++ b/arch/arm64/Kconfig
>> @@ -759,6 +759,8 @@ choice
>>  config CPU_BIG_ENDIAN
>>  	bool "Build big-endian kernel"
>>  	depends on !LD_IS_LLD || LLD_VERSION >= 130000
>> +	# https://github.com/llvm/llvm-project/commit/1379b150991f70a5782e9a143c2ba5308da1161c
>> +	depends on AS_IS_GNU || AS_VERSION >= 150000
>
>Both this backport and the backport of commit e9c6deee00e9 ("arm64: Make
>CPU_BIG_ENDIAN depend on ld.bfd or ld.lld 13.0.0+") are incorrect for
>5.4 and 4.19, which do not have CONFIG_AS_IS_GNU, CONFIG_AS_VERSION, or
>CONFIG_LLD_VERSION.
>
>I don't think this patch is worth pursuing in 5.4 and 4.19, I would be
>very surprised if someone was to hit this issue there, as arm64 big
>endian is pretty esoteric as is. If someone does report something at
>some point, we can consider it then but for now, I think they should be
>dropped from 5.4 and 4.19.

Dropped from 5.4 and 4.19, thanks!

-- 
Thanks,
Sasha