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 1672E2E8DEA; Sun, 6 Sep 2026 00:35:20 +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=1788654922; cv=none; b=cAd1AYjfNneSWcmBzOhAInZVcVTGaXf0Uk/r45bfUrbguFF2AQo5jovxdBU29anP9943jkLbea0hAIhogMn/skJDQbRq5fgXfBNJHVEh3OQmb+0vSFUL7lgoeiWMShWKevGOZieG0c5PyS3m1aapfVrANX/mhnReB80EmukSKG8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788654922; c=relaxed/simple; bh=JPlWoB4DXow6nbg9rJrJMG26hOHZCU/65RpKG60qAgk=; h=Date:To:From:Subject:Message-Id; b=C1ovO8cLTWE1y/QBAyp8ullRjlWqJ9obNNHlJ/rRhArglB54YGUjdO9ReaDMXGXHFCsRmGFOwQni+Bg2BgFaxW2sOZcpxjgCqU20Rdk4zqyV+BPTnOab8RDMh/Et6Vg3O0tFYDhvSQm3/FMzJVeT3DS+XHp3/739E5pKO1tLyYw= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux-foundation.org header.i=@linux-foundation.org header.b=uRSKXqd9; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux-foundation.org header.i=@linux-foundation.org header.b="uRSKXqd9" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 8958A1F00A3A; Sun, 6 Sep 2026 00:35:20 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux-foundation.org; s=korg; t=1788654920; bh=IgY0KNzUHJ/fYTASL01kGfcHFX2ExSozWX8mnJ2vOE4=; h=Date:To:From:Subject; b=uRSKXqd9nlvCMU0q09KdAnE/IRcdC/roaDnVV2U3seKW3ukqu+A7G2Gr/93arujg+ +bDORW2LSDSfCP4rsVikktL70DKfJwcG1ZrnPGgRk18cBUMmv6/2jygkjQ1rLaAz2a eb8HKgi6GCHFHgZe5oCUrg+FiM1YLM3ye3aDfZPk= Date: Sat, 05 Sep 2026 17:35:20 -0700 To: mm-commits@vger.kernel.org,willy@infradead.org,stable@vger.kernel.org,rppt@kernel.org,krystianmkaniewski@gmail.com,akpm@linux-foundation.org From: Andrew Morton Subject: + xarray-fix-index-jumping-backwards-in-xas_find.patch added to mm-hotfixes-unstable branch Message-Id: <20260906003520.8958A1F00A3A@smtp.kernel.org> Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: The patch titled Subject: xarray: fix index jumping backwards in xas_find() has been added to the -mm mm-hotfixes-unstable branch. Its filename is xarray-fix-index-jumping-backwards-in-xas_find.patch This patch will shortly appear at https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/xarray-fix-index-jumping-backwards-in-xas_find.patch This patch will later appear in the mm-hotfixes-unstable branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm Before you just go and hit "reply", please: a) Consider who else should be cc'ed b) Prefer to cc a suitable mailing list as well c) Ideally: find the original patch on the mailing list and do a reply-to-all to that, adding suitable additional cc's *** Remember to use Documentation/process/submit-checklist.rst when testing your code *** The -mm tree is included into linux-next via various branches at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm and is updated there most days ------------------------------------------------------ From: Krystian Kaniewski Subject: xarray: fix index jumping backwards in xas_find() Date: Fri, 4 Sep 2026 14:12:59 +0200 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(). A userspace access that faults in a file-backed mapping can trigger this path. When the index moves backwards, filemap_map_pages() can calculate a PTE outside the page locked for fault-around and dereference a freed page-table page, resulting in a KASAN-detected use-after-free read. 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. Link: https://lore.kernel.org/20260904121301.200049-1-krystianmkaniewski@gmail.com 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=a01c56bd-74d0-411c-afb4-ee6f0cb6cb61 Signed-off-by: Krystian Kaniewski Cc: Matthew Wilcox (Oracle) Cc: Mike Rapoport Cc: Signed-off-by: Andrew Morton --- lib/test_xarray.c | 70 ++++++++++++++++++++++++++++++++++++++++++++ lib/xarray.c | 8 +++-- 2 files changed, 75 insertions(+), 3 deletions(-) --- a/lib/test_xarray.c~xarray-fix-index-jumping-backwards-in-xas_find +++ a/lib/test_xarray.c @@ -1247,6 +1247,75 @@ static noinline void check_multi_find_3( } } +static noinline void check_multi_find_4(struct xarray *xa) +{ +#ifdef CONFIG_XARRAY_MULTI + unsigned int order = XA_CHUNK_SHIFT + 1; + unsigned long next = 1UL << order; + unsigned long start = next - 1; + XA_STATE(xas, xa, start); + XA_STATE_ORDER(split, xa, 0, 0); + void *entry; + unsigned long i; + + /* Multi-index entry in two slots of a non-leaf node. */ + xa_store_order(xa, 0, order, xa_mk_index(0), GFP_KERNEL); + XA_BUG_ON(xa, xa_store_index(xa, next, 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 != start); + + entry = xas_find(&xas, ULONG_MAX); + XA_BUG_ON(xa, entry != xa_mk_index(next)); + XA_BUG_ON(xa, xas.xa_index != next); + + entry = xas_find(&xas, ULONG_MAX); + XA_BUG_ON(xa, entry != NULL); + rcu_read_unlock(); + + xa_erase_index(xa, next); + xa_erase_index(xa, 0); + XA_BUG_ON(xa, !xa_empty(xa)); + + /* Split the multi-index entry after a lookup begins inside it. */ + xa_store_order(xa, 0, order, xa_mk_index(0), GFP_KERNEL); + XA_BUG_ON(xa, xa_store_index(xa, next, GFP_KERNEL) != NULL); + + xas_set(&xas, start); + 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 != start); + rcu_read_unlock(); + + xas_split_alloc(&split, xa_mk_index(0), order, GFP_KERNEL); + if (xas_error(&split)) { + XA_BUG_ON(xa, true); + goto out; + } + xas_lock(&split); + xas_split(&split, xa_mk_index(0), order); + for (i = 0; i < next; i++) + __xa_store(xa, i, xa_mk_index(i), 0); + xas_unlock(&split); + + rcu_read_lock(); + entry = xas_find(&xas, ULONG_MAX); + XA_BUG_ON(xa, entry != xa_mk_index(next)); + XA_BUG_ON(xa, xas.xa_index != next); + + entry = xas_find(&xas, ULONG_MAX); + XA_BUG_ON(xa, entry != NULL); + rcu_read_unlock(); + +out: + 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 +1439,7 @@ static noinline void check_find(struct x 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 */ --- a/lib/xarray.c~xarray-fix-index-jumping-backwards-in-xas_find +++ a/lib/xarray.c @@ -1409,9 +1409,11 @@ void *xas_find(struct xa_state *xas, uns 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); _ Patches currently in -mm which might be from krystianmkaniewski@gmail.com are xarray-fix-index-jumping-backwards-in-xas_find.patch