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[46.135.75.195]) by smtp.gmail.com with ESMTPSA id z24-20020aa7cf98000000b0052341df84d0sm4366470edx.33.2023.08.13.05.33.41 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sun, 13 Aug 2023 05:33:42 -0700 (PDT) From: Mateusz Guzik To: linux-kernel@vger.kernel.org Cc: torvalds@linux-foundation.org, brauner@kernel.org, ebiederm@xmission.com, david@redhat.com, akpm@linux-foundation.org, linux-mm@kvack.org, koct9i@gmail.com, oleg@redhat.com, dave@stgolabs.net, Mateusz Guzik Subject: [PATCH] kernel/fork: stop playing lockless games for exe_file replacement Date: Sun, 13 Aug 2023 14:33:33 +0200 Message-Id: <20230813123333.1705833-1-mjguzik@gmail.com> X-Mailer: git-send-email 2.34.1 MIME-Version: 1.0 X-Rspamd-Queue-Id: 10956100005 X-Rspam-User: X-Rspamd-Server: rspam02 X-Stat-Signature: 567zkait4k5w7oj7hp9my5s7pqgh9z99 X-HE-Tag: 1691930024-85182 X-HE-Meta: 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 vmaZoEFt 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000001, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: xchg originated in 6e399cd144d8 ("prctl: avoid using mmap_sem for exe_file serialization"). While the commit message does not explain *why* the change, clearly the intent was to use mmap_sem less in this codepath. I found the original submission [1] which ultimately claims it cleans things up. However, replacement itself takes it for reading before doing any work and other places associated with it also take it. fe69d560b5bd ("kernel/fork: always deny write access to current MM exe_file") added another lock trip to synchronize the state of exe_file against fork, further defeating the point of xchg. As such I think the atomic here only adds complexity for no benefit. Just write-lock around the replacement. I also note that replacement races against the mapping check loop as nothing synchronizes actual assignment with with said checks but I am not addressing it in this patch. (Is the loop of any use to begin with?) Link: https://lore.kernel.org/linux-mm/1424979417.10344.14.camel@stgolabs.net/ [1] Signed-off-by: Mateusz Guzik Acked-by: Oleg Nesterov --- kernel/fork.c | 14 ++++++-------- 1 file changed, 6 insertions(+), 8 deletions(-) diff --git a/kernel/fork.c b/kernel/fork.c index d2e12b6d2b18..f576ce341e43 100644 --- a/kernel/fork.c +++ b/kernel/fork.c @@ -1464,22 +1464,20 @@ int replace_mm_exe_file(struct mm_struct *mm, struct file *new_exe_file) return ret; } - /* set the new file, lockless */ ret = deny_write_access(new_exe_file); if (ret) return -EACCES; get_file(new_exe_file); - old_exe_file = xchg(&mm->exe_file, new_exe_file); + /* set the new file */ + mmap_write_lock(mm); + old_exe_file = rcu_dereference_raw(mm->exe_file); + rcu_assign_pointer(mm->exe_file, new_exe_file); + mmap_write_unlock(mm); + if (old_exe_file) { - /* - * Don't race with dup_mmap() getting the file and disallowing - * write access while someone might open the file writable. - */ - mmap_read_lock(mm); allow_write_access(old_exe_file); fput(old_exe_file); - mmap_read_unlock(mm); } return 0; }