From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-ed1-f45.google.com (mail-ed1-f45.google.com [209.85.208.45]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id B4D2447CA7D for ; Thu, 27 Aug 2026 14:51:33 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.208.45 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1787842300; cv=none; b=K2+GmK2q8ZdZjSZU/Fc2pEGiSWBM7xpmqtl4Uv57g4b7wCDWZI1GrYRW5md0WpXzPETzD8NLO3vItEwdIpyywIjC2EBNoSEuYYv4Lkh8ravdyoPJz6AdNQ5KBphVGBzTHeJLt4GfaFj5ZrIJmBemjDbapRQ+DpYaJHbIca4QNbU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1787842300; c=relaxed/simple; bh=U85kjlJ/PVaBBqzBCPEmnBBElmV/GlqJmViuguWtnMc=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=s/YpNXO04+Bc+t6ayPGogYTM28bw2ibsd1POmv2hO8OTAPvY9uRUQQ5Gvgl2xvh+zQvgShu/Lhsd2tUAT7fcNJZ0th1uL8fIyZs2pmnjwZoR5xr/RpCEzGIs9JZvmf+CckhNZKPUQ4zuVKbzPrVCf/tO3Joh5oejAOyC79FCFyY= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=gmail.com; spf=pass smtp.mailfrom=gmail.com; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b=VeMRa6U/; arc=none smtp.client-ip=209.85.208.45 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=gmail.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=gmail.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b="VeMRa6U/" Received: by mail-ed1-f45.google.com with SMTP id 4fb4d7f45d1cf-6a5e971c970so1762164a12.0 for ; Thu, 27 Aug 2026 07:51:32 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20251104; t=1787842286; x=1788447086; darn=lists.linux.dev; h=content-transfer-encoding:content-type:in-reply-to:from :content-language:references:cc:to:subject:user-agent:mime-version :date:message-id:from:to:cc:subject:date:message-id:reply-to :content-type; bh=Gc+lqeRuSyvtYVaZ4CHxgk1g9g1Z5ggB5TAlps+bMeE=; b=VeMRa6U/L78KKw9sMa/CgnAPumNgidJzx8Pn3rVauVTDs8jkIJ0Rv/FV5ClsZ7aY3v qvImQJjOOfbPRy/grcjmWGNzXX57rd2WD5C0td5wkcrDHSOvdAZ2XsdLNNUqwF2xt/GC Che+HraEkklA4Z0lBCQyplwleuJMBJQV3w8VG8ehOyN8lWlBGNEI6g+e6Pz7+YN9PXFV iMKZteWeZT01j8ljhXla7sIFmF+2IpQrZUsdvLc15fjTMzT5xGq0am/doX0ACJnoFYJp LymuLLuD6ItjRg9qaE+4t1hJkxaIDnrallUdAi56g+g3oK4pxcRmb68Eso2z1pT1xpU3 MleQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20251104; t=1787842286; x=1788447086; h=content-transfer-encoding:content-type:in-reply-to:from :content-language:references:cc:to:subject:user-agent:mime-version :date:message-id:x-gm-gg:x-gm-message-state:from:to:cc:subject:date :message-id:reply-to:content-type; bh=Gc+lqeRuSyvtYVaZ4CHxgk1g9g1Z5ggB5TAlps+bMeE=; b=gU7KnDGkK153TVBFRhLakV3TSsEcyTPdlDiDC4t6wa6eQ1VN2znglV0xlnlSAlPSzB F0Ts7P3jeWp4kvHkGvEYTnznE4p2U7AEEiLz7X0dr5lHFbFyZgA900PcBoI5m66kCvat rmW0OnbGzGHiaE27H+kiXDrsQej0CPVX8f4mLbAkwIk+tPeT4wLWDAaK90PyjKcL5fmP dna1haBxQ7VPzw6ywzpdNljxR/SYRy8tN/NN859zNmx4xLaEjTj9VstSbTT5norNMXu5 7NFYsnvsMjq2R5GbRMZgIAfe/yak3aqBHV7crWUZQ5sg0VaxUKloOdLLmKaENhXFnrau QYmw== X-Gm-Message-State: AFuF++mjEB+lJaTRlb9j7+UZCc8A7VlD/SN69fo2h+OsDvbViGmGub3J uOI5dl3M0q0TJVdl5uYvBXVU9Gvl4tQNVJkir8RfPJY63u5wxGTfrpUTnf2HNcsIJEc= X-Gm-Gg: AR+sD13zy7Q9SMta1sRsFKTlfe5JoaXsi+QbAD2StdUcEZ0q8WYyB4o7yl/ij99bioE RPC33hH/ddcLi+uCePLz3jYfcxhR9dfRIAwqncnaEn8rviHpTsRkBhweQ401CC1htFlLuhgI1a0 u405z6mFwWFBdDmM2KnVWQxhjYO+VdCYVdKl/xt1/zxFaPBMDskqMwFoRlJp0PGFmDjJElQzIJe 7gaVNY9v+fq5GfNBgmL1TV+/Sn5yMY6+bUd2Rr/W6DpkYy++2aEyL6lsoMOUaa+sfNyNrAZQFlH dpsOw+8HCDlM2zQNWnqYdAXNXaOU2756lOdJPPT11jSc8Qd8dYCGLOJNFTkdbDtPL3ywv+XSCs3 ZOkpYi5ZhRrc1ckgTcbw8U0A/z93qx/p7XtDykAifA/tRJQAz+idu5ediAMqOheMuLUJSFlQmjB TpeL/GMMCowwuLFo0piVomgkUfyil066uv2j1x0Ak6bJJSjqmDSNwVw56wpKPxUktZqpJUmLx2D d45Qo6IxTEmy1EuK5oE0lJBfEiA/xFO7rilvDIJIB8Ofizs7qx3P6k+UcShl9g+YGyIhw7HSec+ uAzybF6BKhj7uvQTJGwNXKMmaOwhsh8hBbUANoUt8MeMHeMNogvCMG9NYFpkAfB7pQ== X-Received: by 2002:a17:907:e1cb:20b0:c25:4835:3ad4 with SMTP id a640c23a62f3a-c2548353f0dmr206609166b.3.1787842286079; Thu, 27 Aug 2026 07:51:26 -0700 (PDT) Received: from [192.168.100.51] (87-205-15-91.static.ip.netia.com.pl. [87.205.15.91]) by smtp.gmail.com with ESMTPSA id a640c23a62f3a-c250a6fd00csm802601166b.15.2026.08.27.07.51.24 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 27 Aug 2026 07:51:24 -0700 (PDT) Message-ID: Date: Thu, 27 Aug 2026 16:51:23 +0200 Precedence: bulk X-Mailing-List: syzbot@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH RFC v2] XArray: fix index jumping backwards in xas_find() To: syzbot , syzkaller-upstream-moderation@googlegroups.com Cc: syzbot@lists.linux.dev References: Content-Language: en-US From: Krystian Kaniewski In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Fix `check_multi_find_4()` so the patch compiles with `CONFIG_TEST_XARRAY=y`. `XA_STATE_ORDER(xas_split, xa, 0, 0)` declares a local object named `xas_split` and hides the `xas_split()` function used later in the same scope. Rename the state, for example to `split`, and update every reference to it. Also pass `xa_mk_index(0)` to `xas_split_alloc()` and `xas_split()` because that is the multi-index entry stored by this test, rather than passing the XArray pointer `xa`. Keep the `lib/xarray.c` cursor repair and both intended `check_multi_find_4()` scenarios unchanged. The non-leaf mismatch must be restored to `get_offset(xas->xa_index, xas->xa_node)` before the existing single `xas_next_offset()` call, while the established leaf calculation remains unchanged. Correct the commit description to match the retained report. It is a KASAN use-after-free read on a page-table page freed through `tlb_remove_table_rcu()`, not a slab use-after-free. Remove the substituted address and stack offsets or quote the retained report accurately. Use the lowercase `xarray:` subject prefix. Preserve the `Fixes`, `Reported-by`, `Closes`, `Link`, and Gemini `Assisted-by` tags. On 8/27/2026 2:31 PM, syzbot wrote: > 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 a use-after-free. > > For example, a KASAN slab-use-after-free read can be triggered in > ptep_get() via filemap_map_pages(): > > BUG: KASAN: slab-use-after-free in ptep_get include/linux/pgtable.h:495 > [inline] > BUG: KASAN: slab-use-after-free in filemap_map_folio_range > mm/filemap.c:3820 [inline] > BUG: KASAN: slab-use-after-free in filemap_map_pages+0x1100/0x2090 > mm/filemap.c:3955 > Read of size 8 at addr ffff888192bdea00 > > Call Trace: > > kasan_report+0x117/0x150 mm/kasan/report.c:595 > ptep_get include/linux/pgtable.h:495 [inline] > filemap_map_folio_range mm/filemap.c:3820 [inline] > filemap_map_pages+0x1100/0x2090 mm/filemap.c:3955 > do_fault_around mm/memory.c:5818 [inline] > do_read_fault mm/memory.c:5851 [inline] > do_fault mm/memory.c:5994 [inline] > do_pte_missing+0x21ac/0x3260 mm/memory.c:4566 > handle_pte_fault mm/memory.c:6379 [inline] > __handle_mm_fault mm/memory.c:6517 [inline] > handle_mm_fault+0x1ad6/0x3070 mm/memory.c:6686 > do_user_addr_fault+0x744/0x1340 arch/x86/mm/fault.c:1394 > handle_page_fault arch/x86/mm/fault.c:1483 [inline] > exc_page_fault+0x6a/0xc0 arch/x86/mm/fault.c:1536 > asm_exc_page_fault+0x26/0x30 arch/x86/include/asm/idtentry.h:595 > > > 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. > > 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 > Link: https://syzkaller.appspot.com/ai_job?id=d9ef07ca-442b-4723-a4ca-51fdb4666218 > To: "Andrew Morton" > To: > To: > To: "Matthew Wilcox" > Cc: > > --- > v2: > - Corrected offset synchronization for non-leaf nodes to use get_offset(xas->xa_index, xas->xa_node). > - Added check_multi_find_4() self-test in lib/test_xarray.c to test multi-index entry lookups and concurrent splitting. > - Updated the commit description to include the KASAN slab-use-after-free report. > > v1: > https://lore.kernel.org/all/e62bb2ac-6ecf-41a2-823f-c13547d5db78@mail.kernel.org/T/ > --- > diff --git a/lib/test_xarray.c b/lib/test_xarray.c > index 5ca0aefee..7d0deccdf 100644 > --- a/lib/test_xarray.c > +++ b/lib/test_xarray.c > @@ -1247,6 +1247,67 @@ static noinline void check_multi_find_3(struct xarray *xa) > } > } > > +static noinline void check_multi_find_4(struct xarray *xa) > +{ > +#ifdef CONFIG_XARRAY_MULTI > + XA_STATE(xas, xa, 100); > + XA_STATE_ORDER(xas_split, xa, 0, 0); > + void *entry; > + unsigned long i; > + > + /* (1) Order-7 entry (0-127) with adjacent entry at 128, starting at 100 */ > + xa_store_order(xa, 0, 7, xa_mk_index(0), GFP_KERNEL); > + XA_BUG_ON(xa, xa_store_index(xa, 128, GFP_KERNEL) != NULL); > + > + rcu_read_lock(); > + entry = xas_find(&xas, ULONG_MAX); > + XA_BUG_ON(xa, entry != xa_mk_index(0)); > + XA_BUG_ON(xa, xas.xa_index != 100); > + > + entry = xas_find(&xas, ULONG_MAX); > + XA_BUG_ON(xa, entry != xa_mk_index(128)); > + XA_BUG_ON(xa, xas.xa_index != 128); > + > + entry = xas_find(&xas, ULONG_MAX); > + XA_BUG_ON(xa, entry != NULL); > + rcu_read_unlock(); > + > + xa_erase_index(xa, 128); > + xa_erase_index(xa, 0); > + XA_BUG_ON(xa, !xa_empty(xa)); > + > + /* (2) Splitting a multi-index entry after a lookup begins inside it */ > + xa_store_order(xa, 0, 7, xa_mk_index(0), GFP_KERNEL); > + XA_BUG_ON(xa, xa_store_index(xa, 128, GFP_KERNEL) != NULL); > + > + xas_set(&xas, 100); > + rcu_read_lock(); > + entry = xas_find(&xas, ULONG_MAX); > + XA_BUG_ON(xa, entry != xa_mk_index(0)); > + XA_BUG_ON(xa, xas.xa_index != 100); > + rcu_read_unlock(); > + > + xas_split_alloc(&xas_split, xa, 7, GFP_KERNEL); > + xas_lock(&xas_split); > + xas_split(&xas_split, xa, 7); > + for (i = 0; i < 128; i++) > + __xa_store(xa, i, xa_mk_index(i), 0); > + xas_unlock(&xas_split); > + > + rcu_read_lock(); > + entry = xas_find(&xas, ULONG_MAX); > + XA_BUG_ON(xa, entry != xa_mk_index(128)); > + XA_BUG_ON(xa, xas.xa_index != 128); > + > + entry = xas_find(&xas, ULONG_MAX); > + XA_BUG_ON(xa, entry != NULL); > + rcu_read_unlock(); > + > + xa_destroy(xa); > + XA_BUG_ON(xa, !xa_empty(xa)); > +#endif > +} > + > static noinline void check_find_1(struct xarray *xa) > { > unsigned long i, j, k; > @@ -1370,6 +1431,7 @@ static noinline void check_find(struct xarray *xa) > check_multi_find_1(xa, i); > check_multi_find_2(xa); > check_multi_find_3(xa); > + check_multi_find_4(xa); > } > > /* See find_swap_entry() in mm/shmem.c */ > diff --git a/lib/xarray.c b/lib/xarray.c > index 9a8b49165..980324d68 100644 > --- a/lib/xarray.c > +++ b/lib/xarray.c > @@ -1406,9 +1406,11 @@ void *xas_find(struct xa_state *xas, unsigned long max) > entry = xas_load(xas); > if (entry || xas_not_node(xas->xa_node)) > return entry; > - } else if (!xas->xa_node->shift && > - xas->xa_offset != (xas->xa_index & XA_CHUNK_MASK)) { > - xas->xa_offset = ((xas->xa_index - 1) & XA_CHUNK_MASK) + 1; > + } else if (xas->xa_offset != get_offset(xas->xa_index, xas->xa_node)) { > + if (!xas->xa_node->shift) > + xas->xa_offset = ((xas->xa_index - 1) & XA_CHUNK_MASK) + 1; > + else > + xas->xa_offset = get_offset(xas->xa_index, xas->xa_node); > } > > xas_next_offset(xas); > > > base-commit: 8d3ae59288f1e7d58d76558a6ee96d533bc5019f