Re: FAILED: Patch "x86/uprobes: Fix XOL allocation failure for 32-bit tasks" failed to apply to 6.18-stable tree

[Oleg Nesterov <oleg@redhat.com>]

On 02/28, Sasha Levin wrote:
>
> The patch below does not apply to the 6.18-stable tree.
> If someone wants it applied there, or to any other stable or longterm
> tree, then please email the backport, including the original git commit
> id to <stable@vger.kernel.org>.

I never know how to react to "failed to apply to stabe" emails. I am going
to send [PATCH 6.18.y] in reply to this email.

Is it OK?

Oleg.

> Thanks,
> Sasha
> 
> ------------------ original commit in Linus's tree ------------------
> 
> From d55c571e4333fac71826e8db3b9753fadfbead6a Mon Sep 17 00:00:00 2001
> From: Oleg Nesterov <oleg@redhat.com>
> Date: Sun, 11 Jan 2026 16:00:37 +0100
> Subject: [PATCH] x86/uprobes: Fix XOL allocation failure for 32-bit tasks
> 
> This script
> 
> 	#!/usr/bin/bash
> 
> 	echo 0 > /proc/sys/kernel/randomize_va_space
> 
> 	echo 'void main(void) {}' > TEST.c
> 
> 	# -fcf-protection to ensure that the 1st endbr32 insn can't be emulated
> 	gcc -m32 -fcf-protection=branch TEST.c -o test
> 
> 	bpftrace -e 'uprobe:./test:main {}' -c ./test
> 
> "hangs", the probed ./test task enters an endless loop.
> 
> The problem is that with randomize_va_space == 0
> get_unmapped_area(TASK_SIZE - PAGE_SIZE) called by xol_add_vma() can not
> just return the "addr == TASK_SIZE - PAGE_SIZE" hint, this addr is used
> by the stack vma.
> 
> arch_get_unmapped_area_topdown() doesn't take TIF_ADDR32 into account and
> in_32bit_syscall() is false, this leads to info.high_limit > TASK_SIZE.
> vm_unmapped_area() happily returns the high address > TASK_SIZE and then
> get_unmapped_area() returns -ENOMEM after the "if (addr > TASK_SIZE - len)"
> check.
> 
> handle_swbp() doesn't report this failure (probably it should) and silently
> restarts the probed insn. Endless loop.
> 
> I think that the right fix should change the x86 get_unmapped_area() paths
> to rely on TIF_ADDR32 rather than in_32bit_syscall(). Note also that if
> CONFIG_X86_X32_ABI=y, in_x32_syscall() falsely returns true in this case
> because ->orig_ax = -1.
> 
> But we need a simple fix for -stable, so this patch just sets TS_COMPAT if
> the probed task is 32-bit to make in_ia32_syscall() true.
> 
> Fixes: 1b028f784e8c ("x86/mm: Introduce mmap_compat_base() for 32-bit mmap()")
> Reported-by: Paulo Andrade <pandrade@redhat.com>
> Signed-off-by: Oleg Nesterov <oleg@redhat.com>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> Link: https://lore.kernel.org/all/aV5uldEvV7pb4RA8@redhat.com/
> Cc: stable@vger.kernel.org
> Link: https://patch.msgid.link/aWO7Fdxn39piQnxu@redhat.com
> ---
>  arch/x86/kernel/uprobes.c | 24 ++++++++++++++++++++++++
>  include/linux/uprobes.h   |  1 +
>  kernel/events/uprobes.c   | 10 +++++++---
>  3 files changed, 32 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kernel/uprobes.c b/arch/x86/kernel/uprobes.c
> index 7be8e361ca55b..619dddf54424e 100644
> --- a/arch/x86/kernel/uprobes.c
> +++ b/arch/x86/kernel/uprobes.c
> @@ -1823,3 +1823,27 @@ bool is_uprobe_at_func_entry(struct pt_regs *regs)
>  
>  	return false;
>  }
> +
> +#ifdef CONFIG_IA32_EMULATION
> +unsigned long arch_uprobe_get_xol_area(void)
> +{
> +	struct thread_info *ti = current_thread_info();
> +	unsigned long vaddr;
> +
> +	/*
> +	 * HACK: we are not in a syscall, but x86 get_unmapped_area() paths
> +	 * ignore TIF_ADDR32 and rely on in_32bit_syscall() to calculate
> +	 * vm_unmapped_area_info.high_limit.
> +	 *
> +	 * The #ifdef above doesn't cover the CONFIG_X86_X32_ABI=y case,
> +	 * but in this case in_32bit_syscall() -> in_x32_syscall() always
> +	 * (falsely) returns true because ->orig_ax == -1.
> +	 */
> +	if (test_thread_flag(TIF_ADDR32))
> +		ti->status |= TS_COMPAT;
> +	vaddr = get_unmapped_area(NULL, TASK_SIZE - PAGE_SIZE, PAGE_SIZE, 0, 0);
> +	ti->status &= ~TS_COMPAT;
> +
> +	return vaddr;
> +}
> +#endif
> diff --git a/include/linux/uprobes.h b/include/linux/uprobes.h
> index ee3d36eda45dd..f548fea2adec8 100644
> --- a/include/linux/uprobes.h
> +++ b/include/linux/uprobes.h
> @@ -242,6 +242,7 @@ extern void arch_uprobe_clear_state(struct mm_struct *mm);
>  extern void arch_uprobe_init_state(struct mm_struct *mm);
>  extern void handle_syscall_uprobe(struct pt_regs *regs, unsigned long bp_vaddr);
>  extern void arch_uprobe_optimize(struct arch_uprobe *auprobe, unsigned long vaddr);
> +extern unsigned long arch_uprobe_get_xol_area(void);
>  #else /* !CONFIG_UPROBES */
>  struct uprobes_state {
>  };
> diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
> index a7d7d83ca1d78..dfbce021fb027 100644
> --- a/kernel/events/uprobes.c
> +++ b/kernel/events/uprobes.c
> @@ -1694,6 +1694,12 @@ static const struct vm_special_mapping xol_mapping = {
>  	.mremap = xol_mremap,
>  };
>  
> +unsigned long __weak arch_uprobe_get_xol_area(void)
> +{
> +	/* Try to map as high as possible, this is only a hint. */
> +	return get_unmapped_area(NULL, TASK_SIZE - PAGE_SIZE, PAGE_SIZE, 0, 0);
> +}
> +
>  /* Slot allocation for XOL */
>  static int xol_add_vma(struct mm_struct *mm, struct xol_area *area)
>  {
> @@ -1709,9 +1715,7 @@ static int xol_add_vma(struct mm_struct *mm, struct xol_area *area)
>  	}
>  
>  	if (!area->vaddr) {
> -		/* Try to map as high as possible, this is only a hint. */
> -		area->vaddr = get_unmapped_area(NULL, TASK_SIZE - PAGE_SIZE,
> -						PAGE_SIZE, 0, 0);
> +		area->vaddr = arch_uprobe_get_xol_area();
>  		if (IS_ERR_VALUE(area->vaddr)) {
>  			ret = area->vaddr;
>  			goto fail;
> -- 
> 2.51.0
> 
> 
> 
>