[PATCHv2] arm64/entry: Fix arm64-specific rseq brokenness

[Catalin Marinas]

On Fri, May 08, 2026 at 03:20:23PM +0100, Mark Rutland wrote:
> Mathias Stearn reports that since v6.19, there are two big issues
> affecting rseq:
> 
> (1) On arm64 specifically, rseq critical sections aren't aborted when
>     they should be.
> 
> (2) The 'cpu_id_start' field is no longer written by the kernel in all
>     cases it used to be, including some cases where TCMalloc depends on
>     the kernel clobbering the field.
> 
> This patch fixes issue #1. This patch DOES NOT fix issue #2, which will
> need to be addressed by other patches.
> 
> The arm64-specific brokenness is a result of commits:
> 
>   2fc0e4b4126c ("rseq: Record interrupt from user space")
>   39a167560a61 ("rseq: Optimize event setting")
> 
> The first commit failed to add a call to rseq_note_user_irq_entry() on
> arm64. Thus arm64 never sets rseq_event::user_irq to record that it may
> be necessary to abort an active rseq critical section upon return to
> userspace. On its own, this commit had no functional impact as the value
> of rseq_event::user_irq was not consumed.
> 
> The second commit relied upon rseq_event::user_irq to determine whether
> or not to bother to perform rseq work when returning to userspace. As
> rseq_event::user_irq wasn't set on arm64, this work would be skipped,
> and consequently an active rseq critical section would not be aborted.
> 
> Fix this by giving arm64 syscall-specific entry/exit paths, and
> performing the relevant logic in syscall and non-syscall paths,
> including calling rseq_note_user_irq_entry() for non-syscall entry.
> 
> Currently arm64 cannot use syscall_enter_from_user_mode(),
> syscall_exit_to_user_mode(), and irqentry_exit_to_user_mode(), due to
> ordering constraints with exception masking, and risk of ABI breakage
> for syscall tracing/audit/etc. For the moment the entry/exit logic is
> left as arm64-specific, directly using enter_from_user_mode() and
> exit_to_user_mode(), but mirroring the generic code.
> 
> I intend to follow up with refactoring/cleanup, as we did for kernel
> mode entry paths in commit:
> 
>   041aa7a85390 ("entry: Split preemption from irqentry_exit_to_kernel_mode()")
> 
> ... which will allow arm64 to use the GENERIC_IRQ_ENTRY functions directly.
> 
> Fixes: 39a167560a61 ("rseq: Optimize event setting")
> Reported-by: Mathias Stearn <mathias at mongodb.com>
> Link: https://lore.kernel.org/regressions/CAHnCjA25b+nO2n5CeifknSKHssJpPrjnf+dtr7UgzRw4Zgu=oA@mail.gmail.com/
> Signed-off-by: Mark Rutland <mark.rutland at arm.com>
> Cc: Catalin Marinas <catalin.marinas at arm.com>
> Cc: Chris Kennelly <ckennelly at google.com>
> Cc: Dmitry Vyukov <dvyukov at google.com>
> Cc: Jinjie Ruan <ruanjinjie at huawei.com>
> Cc: Mathieu Desnoyers <mathieu.desnoyers at efficios.com>
> Cc: Peter Zijlstra <peterz at infradead.org>
> Cc: Thomas Gleixner <tglx at linutronix.de>
> Cc: Will Deacon <will at kernel.org>

Acked-by: Catalin Marinas <catalin.marinas at arm.com>