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Received: from willy by casper.infradead.org with local (Exim 4.94.2 #2 (Red Hat Linux)) id 1poymP-00Cw0o-J5; Wed, 19 Apr 2023 03:46:49 +0000 Date: Wed, 19 Apr 2023 04:46:49 +0100 From: Matthew Wilcox To: Waiman Long Cc: Andrew Morton , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Joe Mario , Barry Marson , Rafael Aquini Subject: Re: [PATCH] mm/mmap: Map MAP_STACK to VM_STACK Message-ID: References: <20230418210230.3495922-1-longman@redhat.com> <20230418141852.75e551e57e97f4b522957c5c@linux-foundation.org> <6c3c68b1-c4d4-dd82-58e8-f7013fb6c8e5@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <6c3c68b1-c4d4-dd82-58e8-f7013fb6c8e5@redhat.com> X-Rspam-User: X-Rspamd-Server: rspam03 X-Stat-Signature: q37tm8aqbkf9r9u7p6jy6uwnbxt9suwa X-Rspamd-Queue-Id: 1A90A1C0003 X-HE-Tag: 1681876012-80924 X-HE-Meta: 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 76liXQRW STjP8bq4aF/MXweAQ01bON6s6KCDq+guViZtt+lh+pafEIB2UuGTJE47oKohZtwizjJ/RjukrEZcbjkdd46G8afY/8gjGOWGx9t15SSNpeyGwhTqL3CB+5kmEDZANAHjP4wzi7ck21ETkb/rifyJ05eQcImsqSGIJGL+i1Vdn6U4hAsTzAUTbSQ6ys6clyY1W0DcrOnZCeCThiSKwTtfRXRzrteAUWZQVbclz X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Tue, Apr 18, 2023 at 09:16:37PM -0400, Waiman Long wrote: >  1) App runs creating lots of threads. >  2) It mmap's 256K pages of anonymous memory. >  3) It writes executable code to that memory. >  4) It calls mprotect() with PROT_EXEC on that memory so >     it can subsequently execute the code. > > The above mprotect() will fail if the mmap'd region's VMA gets merged with > the VMA for one of the thread stacks.  That's because the default RHEL > SELinux policy is to not allow executable stacks. By the way, this is a daft policy. The policy you really want is EXEC|WRITE is not allowed. A non-writable stack is useless, so it's actually a superset of your current policy. Forbidding _simultaneous_ write and executable is just good programming. This way, you don't need to care about the underlying VMA's current permissions, you just need to do: if ((prot & (PROT_EXEC|PROT_WRITE)) == (PROT_EXEC|PROT_WRITE)) return -EACCESS;