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Received: from willy by casper.infradead.org with local (Exim 4.94.2 #2 (Red Hat Linux)) id 1pQC2q-002Qfo-Vn; Thu, 09 Feb 2023 18:53:21 +0000 Date: Thu, 9 Feb 2023 18:53:20 +0000 From: Matthew Wilcox To: David Chen Cc: "linux-kernel@vger.kernel.org" , Andrew Morton , "linux-mm@kvack.org" , "stable@vger.kernel.org" Subject: Re: [PATCH] Fix page corruption caused by racy check in __free_pages Message-ID: References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspamd-Queue-Id: E811B1C0007 X-Stat-Signature: 694esse7e9tpeccpnyehbx4bczfkcnhf X-Rspam-User: X-Rspamd-Server: rspam08 X-HE-Tag: 1675968811-303747 X-HE-Meta: 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 FoS8Z00n 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 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 Thu, Feb 09, 2023 at 05:48:28PM +0000, David Chen wrote: > So the problem is the check PageHead is racy because at this point we > already dropped our reference to the page. So even if we came in with > compound page, the page can already be freed and PageHead can return > false and we will end up freeing all the tail pages causing double free. > > Fixes: e320d3012d25 ("mm/page_alloc.c: fix freeing non-compound pages") > Cc: Andrew Morton > Cc: Matthew Wilcox (Oracle) > Cc: linux-mm@kvack.org > Cc: stable@vger.kernel.org > Signed-off-by: Chunwei Chen Reviewed-by: Matthew Wilcox (Oracle) That's a pretty narrow window you managed to hit! Just a few instructions wide. I guess that answers the question Andrew originally had (Does this ever happen?) I just changed it from a silent memory leak into a double-free. > --- > mm/page_alloc.c | 5 ++++- > 1 file changed, 4 insertions(+), 1 deletion(-) > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c > index 0745aedebb37..3bb3484563ed 100644 > --- a/mm/page_alloc.c > +++ b/mm/page_alloc.c > @@ -5631,9 +5631,12 @@ EXPORT_SYMBOL(get_zeroed_page); > */ > void __free_pages(struct page *page, unsigned int order) > { > + /* get PageHead before we drop reference */ > + int head = PageHead(page); > + > if (put_page_testzero(page)) > free_the_page(page, order); > - else if (!PageHead(page)) > + else if (!head) > while (order-- > 0) > free_the_page(page + (1 << order), order); > } > -- > 2.22.3