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[87.205.15.91]) by smtp.gmail.com with ESMTPSA id 4fb4d7f45d1cf-6a7e6bf337bsm1063392a12.26.2026.09.04.05.16.32 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 04 Sep 2026 05:16:32 -0700 (PDT) Message-ID: <493efefa-308b-4d3d-a6e9-841a3b118283@gmail.com> Date: Fri, 4 Sep 2026 14:16:31 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] xarray: fix index jumping backwards in xas_find() From: Krystian Kaniewski To: Mike Rapoport Cc: Andrew Morton , syzbot , syzkaller-bugs@googlegroups.com, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, Matthew Wilcox , linux-kernel@vger.kernel.org, syzbot@lists.linux.dev References: <2992424b-2120-489e-9010-f45f46ed52c8@mail.kernel.org> <20260903112251.6114f91af953412d2355e5b3@linux-foundation.org> Content-Language: en-US In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit I have fixed the test issue and submitted patch as v2. I'm sorry about issue v1 caused, hopefully v2 is going to be fully correct now. On 9/4/2026 10:41 AM, Krystian Kaniewski wrote: > Hello, > > Thanks for running tests on my patch. I've quickly looked into the test > result and the problem seems to be related to check_multi_find_4() test and > not the fix itself. I'll send a v2 patch once I get the test fixed. > > pt., 4 wrz 2026 o 10:22 Mike Rapoport napisał(a): > >> 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. >>