From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 885A93B8407 for ; Fri, 11 Sep 2026 09:53:08 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789120389; cv=none; b=ECe0OKmQsgOtogUK/ruy070biwjZfQgsHul2TpIiKC/ThlHultvgDcxlbL5+Lt8u/5Ap1BSPDmeCrq21MhziAAU9+PR4LqGYBiKAHJf/GxDDA0+B6laFBJeNLkTQgIO5fTIk9df9YnHyXrrFq07jlLUr6f71EvHqQrHAryvVGC0= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789120389; c=relaxed/simple; bh=9Q0FyA/AYPrvOMElEg2mr5PS58WF1MuPYMx9/TvJ0DQ=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=VtCSVSJnnzk5HxB3IbvDLXVLp/NeUTJV2AGMQ6o891V0spqkRq1X7xMpqx5jLSg8WyQkIv8irdVEbQvdMQGAQ/8oYsrErO9Wi0E2bTDhMJtO7OTeK+EzRTnQy87APvlm+LP8DF7yVpx5BvNtFagjkoBMgRjOxqASqYYYehdS8is= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=c0IeisZj; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="c0IeisZj" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 1AF861F000FF; Fri, 11 Sep 2026 09:53:05 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1789120388; bh=N9ehvSphLCnbyvmeF6JnfHpcqPMKJTQjg3on9HWu+eo=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=c0IeisZj04XLHA+Ib63VvfSrKsewBlhudaADiRtYA/vUAMSYpyN+v6tTK0WPl4TiH T58BYGoTwtbvGG4e90I4Q6SFD2HEh1LCsGSt6DioxSloqO2IBzArmakSffMdEBUsY5 VJaIdNVjsPrsvKa03dN7MwXxBSsOEDuEd11WN+c8KudivVvzuHGaRcMoAQ84kKlPK3 enLFDVheASTJl2QR5+t15rG5IQHcKz2/gIAiEs/pKStzyeUerkxH/ckGS82vSKFjW+ 8CnVjr3nfbGlBBqVBkZE3Bvat8VihOFdTSVTmZafzYHp1HGeOtYRMAr+0jNcSs+JV2 29wRPeRjZr6xg== Date: Fri, 11 Sep 2026 10:53:03 +0100 From: "Lorenzo Stoakes (ARM)" To: Sasha Levin Cc: stable@vger.kernel.org, syzbot+f12658786a4153df5113@syzkaller.appspotmail.com, "Vlastimil Babka (SUSE)" , Kunwu Chan , Pedro Falcato , Jann Horn , "Liam R. Howlett" , Li Xinhai , Andrew Morton Subject: Re: [PATCH 7.2.y] mm/mremap: reset unfaulted VMA page offset for MREMAP_DONTUNMAP Message-ID: References: <2026090956-sampling-underpay-4fb9@gregkh> <20260911011746.4048905-1-sashal@kernel.org> Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20260911011746.4048905-1-sashal@kernel.org> On Thu, Sep 10, 2026 at 09:17:46PM -0400, Sasha Levin wrote: > From: "Lorenzo Stoakes (ARM)" > > [ Upstream commit 35b0fb391b0df57383bc15985bb769f4555c97ba ] > > Uniquely an mremap() invocation using the MREMAP_DONTUNMAP flag can reset > a faulted VMA into an unfaulted one. > > It does so after the page tables have been moved to the copied VMA with > MREMAP_DONTUNMAP leaving the old VMA in place which is naturally unfaulted > as the page tables it had are no longer present. > > However, in doing so, it violates the invariant that the anonymous page > offset of an unfaulted VMA is vma->vm_start >> PAGE_SHIFT. > > This is because a VMA may have been faulted in, mremap()'d (causing a > delta between its page offset and vma->vm_start >> PAGE_SHIFT), and then > mremap()'d again with MREMAP_DONTUNMAP resulting in the unfaulting. > > This condition is a violation of a fundamental assumption in mm, but now > also triggers an assert in assert_sane_pgoff() which explicitly checks for > this condition. > > Correct it by resetting the VMA's page offset at the point of completing > the MREMAP_DONTUNMAP operation. > > Link: https://lore.kernel.org/20260825-fix-mremap-dontunmap-pgoff-v1-1-39a40b2c98b3@kernel.org > Fixes: 1583aa278f5f ("mm: mremap: unlink anon_vmas when mremap with MREMAP_DONTUNMAP success") > Signed-off-by: Lorenzo Stoakes (ARM) > Reported-by: syzbot+f12658786a4153df5113@syzkaller.appspotmail.com > Closes: https://lore.kernel.org/all/6a87853b.ae6ddae5.3da009.0023.GAE@google.com/ > Tested-by: syzbot+f12658786a4153df5113@syzkaller.appspotmail.com > Acked-by: Vlastimil Babka (SUSE) > Reviewed-by: Kunwu Chan > Reviewed-by: Pedro Falcato > Cc: Jann Horn > Cc: Liam R. Howlett > Cc: Li Xinhai > Cc: > Signed-off-by: Andrew Morton > [ adapted VMA page-offset helpers to use the branch’s single vm_pgoff field. ] > Signed-off-by: Sasha Levin LGTM thanks! > --- > mm/mremap.c | 21 ++++++++++++++++----- > 1 file changed, 16 insertions(+), 5 deletions(-) > > diff --git a/mm/mremap.c b/mm/mremap.c > index e9c8b1d05832b..a7bd785e603f3 100644 > --- a/mm/mremap.c > +++ b/mm/mremap.c > @@ -1321,18 +1321,29 @@ static void dontunmap_complete(struct vma_remap_struct *vrm, > { > unsigned long start = vrm->addr; > unsigned long end = vrm->addr + vrm->old_len; > - unsigned long old_start = vrm->vma->vm_start; > - unsigned long old_end = vrm->vma->vm_end; > + struct vm_area_struct *vma = vrm->vma; > + unsigned long old_start = vma->vm_start; > + unsigned long old_end = vma->vm_end; > > /* We always clear VM_LOCKED[ONFAULT] on the old VMA. */ > - vm_flags_clear(vrm->vma, VM_LOCKED_MASK); > + vm_flags_clear(vma, VM_LOCKED_MASK); > > /* > * anon_vma links of the old vma is no longer needed after its page > * table has been moved. > */ > - if (new_vma != vrm->vma && start == old_start && end == old_end) > - unlink_anon_vmas(vrm->vma); > + if (new_vma != vma && start == old_start && end == old_end) { > + const pgoff_t pgoff_unfaulted = vma->vm_start >> PAGE_SHIFT; > + > + unlink_anon_vmas(vma); > + /* > + * The VMA is now unfaulted and it is an invariant that > + * unfaulted anonymous VMAs have page offset equal to > + * vma->vm_start >> PAGE_SHIFT. > + */ > + if (vma_is_anonymous(vma) && !vma->vm_file) > + vma->vm_pgoff = pgoff_unfaulted; > + } > > /* Because we won't unmap we don't need to touch locked_vm. */ > } > -- > 2.53.0 > -- Cheers, Lorenzo