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 A40953DB327; Fri, 4 Sep 2026 08:22:01 +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=1788510122; cv=none; b=fdw29zOUsr9xGkg6LhXKh6IEacyrGIs8idurPXcwAt43vfglttGiqOnDbVLpNmVisWriXoL/crmGMhnDhbF1HyIRZ4HH8BgeOoSjJNrXtcA30A2H0d5W1ZWdalSR5aY47iRQkuzaD5kD1B4Tz7YCYdQWUXPbHGd/LZxEIZvdM1E= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788510122; c=relaxed/simple; bh=NOPnmzYQ2FNNWOW/u/n3R8m2vKkoaoqBEYdskMu4U3M=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=T0+++Dwy/gORnvm1XBUF1Vs+JcyaeLYtJj3G5z+kBbu7ksMCt2xmkbb3vMSW8GCyUljCXCM44cQ2JIHB5ou3/Fl0tTCR5Ihu5uQ9r2joGbSsdi6heZwglbxpO3v0M0PVizAlOEVsEHdewcn/rk5Lyf6bT9aomNJ8H8VA1aGDIzA= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=OjIgCijT; 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="OjIgCijT" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 079581F00A3D; Fri, 4 Sep 2026 08:21:58 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1788510121; bh=ViqxEg2nic21/cC+OdaYhidCKvzIE/91TrHsQDV+MNs=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=OjIgCijTg8xubFCjXgjFEtmB1OQ+cfvyNFkyhkMtj8Y7nIE6k+3q/RspG0eSdf5bs 2svaQZbbt+haYCBqjLTdpFHywBlL3Dv1AeDUK1p18/QTS9oyZ4cnvAmfa3nBgsmhQM tD6TyWTRFWOZXPJgDpzltMHLopfSdJDEIj0ME57ePoF44PluyT27F2LO0m3wSN+vTx wWntQnd3eH+IaFBkfmKkYvd1h5gGdIK9gnQOyZ7ZeyPE6KjCBYBxL+XNERadmabR4i RSjSNiRJzetnE1zaqPaJTtGeDy75H3nSCS09VbBvslDUSLXNf+yQesncXhmSVvL+5F ZVxAtzIYZMaVg== Date: Fri, 4 Sep 2026 11:21:55 +0300 From: Mike Rapoport To: Andrew Morton Cc: syzbot , syzkaller-bugs@googlegroups.com, Krystian Kaniewski , linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, Matthew Wilcox , linux-kernel@vger.kernel.org, syzbot@lists.linux.dev Subject: Re: [PATCH] xarray: fix index jumping backwards in xas_find() Message-ID: References: <2992424b-2120-489e-9010-f45f46ed52c8@mail.kernel.org> <20260903112251.6114f91af953412d2355e5b3@linux-foundation.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260903112251.6114f91af953412d2355e5b3@linux-foundation.org> On Thu, Sep 03, 2026 at 11:22:51AM -0700, Andrew Morton wrote: > On Thu, 3 Sep 2026 13:48:55 +0000 (UTC) "syzbot" wrote: > > > From: Krystian Kaniewski > > > > A bug in the XArray iterator xas_find() causes the iterator's index > > (xas->xa_index) to jump backwards when iterating over a multi-index entry > > (like a THP) that resides in a non-leaf node and is concurrently split. > > > > When iterating over a multi-index entry in a non-leaf node, xas_load() sets > > xas->xa_offset to the base offset of the entry, but leaves xas->xa_index at > > the requested index. When the caller subsequently wants to advance to the > > next entry, xas_find() is called. xas_find() attempts to synchronize > > xas->xa_offset with xas->xa_index before advancing. However, the fixup > > logic was incorrectly restricted to leaf nodes (!xas->xa_node->shift). > > Because the THP resides in a non-leaf node, the fixup is skipped. > > > > As a result, xas_find() simply increments xas->xa_offset and recalculates > > xas->xa_index based on this new offset. This causes xas->xa_index to jump > > backwards. If the THP was concurrently split, the entry at the new offset > > is a node pointer, so xas_find() descends into it and returns the folio at > > the backwards index. The caller (filemap_map_pages()) then calculates the > > PTE pointer based on this backwards index, resulting in an invalid memory > > access such as an out-of-bounds read or use-after-free on a page-table page > > freed via tlb_remove_table_rcu(). > > > > To fix this, check if xas->xa_offset matches get_offset(xas->xa_index, > > xas->xa_node). If it does not and the node is a non-leaf node, set > > xas->xa_offset to get_offset(xas->xa_index, xas->xa_node) before advancing. > > Also add test cases in test_xarray to verify xas_find() behavior when > > iterating over and splitting multi-index entries. > > Thanks. But the changelog omits vital information: a description of > the userspace-visible runtime effects of the bug. > > > Fixes: b803b42823d0 ("xarray: Add XArray iterators") > > Assisted-by: Gemini:gemini-3.7-flash syzbot > > Reported-by: syzbot+b72767277f29b6407083@syzkaller.appspotmail.com > > Closes: https://syzkaller.appspot.com/bug?extid=b72767277f29b6407083 > > I see from there that it was a use-after-free, detected by KASAN. I'll > update the changelog and I'll add cc:stable. > > I'll queue the patch for testing while awaiting review input. The testing shows there are issues with this patch: https://github.com/linux-mm/linux-mm/actions/runs/33815757662/job/100847425973 And since it's in mm-unstable now it breaks all CI jobs :( > > Link: https://syzkaller.appspot.com/ai_job?id=a01c56bd-74d0-411c-afb4-ee6f0cb6cb61 > > -- Sincerely yours, Mike.