53 lines
2.1 KiB
Diff
53 lines
2.1 KiB
Diff
From: Linus Torvalds <torvalds@linux-foundation.org>
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Date: Thu, 11 Apr 2019 10:06:20 -0700
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Subject: mm: make page ref count overflow check tighter and more explicit
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Origin: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable.git/commit?id=9f6da5fd05577ef4a05c1744cc7098d0173823af
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Bug-Debian-Security: https://security-tracker.debian.org/tracker/CVE-2019-11487
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commit f958d7b528b1b40c44cfda5eabe2d82760d868c3 upstream.
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We have a VM_BUG_ON() to check that the page reference count doesn't
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underflow (or get close to overflow) by checking the sign of the count.
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That's all fine, but we actually want to allow people to use a "get page
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ref unless it's already very high" helper function, and we want that one
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to use the sign of the page ref (without triggering this VM_BUG_ON).
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Change the VM_BUG_ON to only check for small underflows (or _very_ close
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to overflowing), and ignore overflows which have strayed into negative
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territory.
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Acked-by: Matthew Wilcox <willy@infradead.org>
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Cc: Jann Horn <jannh@google.com>
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Cc: stable@kernel.org
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Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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---
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include/linux/mm.h | 6 +++++-
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1 file changed, 5 insertions(+), 1 deletion(-)
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diff --git a/include/linux/mm.h b/include/linux/mm.h
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index e899460f1bc5..9965704813dc 100644
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--- a/include/linux/mm.h
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+++ b/include/linux/mm.h
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@@ -915,6 +915,10 @@ static inline bool is_device_public_page(const struct page *page)
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}
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#endif /* CONFIG_DEV_PAGEMAP_OPS */
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+/* 127: arbitrary random number, small enough to assemble well */
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+#define page_ref_zero_or_close_to_overflow(page) \
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+ ((unsigned int) page_ref_count(page) + 127u <= 127u)
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+
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static inline void get_page(struct page *page)
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{
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page = compound_head(page);
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@@ -922,7 +926,7 @@ static inline void get_page(struct page *page)
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* Getting a normal page or the head of a compound page
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* requires to already have an elevated page->_refcount.
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*/
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- VM_BUG_ON_PAGE(page_ref_count(page) <= 0, page);
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+ VM_BUG_ON_PAGE(page_ref_zero_or_close_to_overflow(page), page);
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page_ref_inc(page);
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}
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