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Kemerlis" , Juerg Haefliger , David Woodhouse Subject: [RFC PATCH v9 11/13] xpfo, mm: optimize spinlock usage in xpfo_kunmap Date: Wed, 3 Apr 2019 11:34:12 -0600 Message-Id: <5bab13e12d4215112ad2180106cc6bb9b513754a.1554248002.git.khalid.aziz@oracle.com> X-Mailer: git-send-email 2.17.1 In-Reply-To: References: In-Reply-To: References: X-Proofpoint-Virus-Version: vendor=nai engine=5900 definitions=9216 signatures=668685 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 suspectscore=0 malwarescore=0 phishscore=0 bulkscore=0 spamscore=0 mlxscore=0 mlxlogscore=999 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1810050000 definitions=main-1904030118 X-Proofpoint-Virus-Version: vendor=nai engine=5900 definitions=9216 signatures=668685 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 priorityscore=1501 malwarescore=0 suspectscore=0 phishscore=0 bulkscore=0 spamscore=0 clxscore=1011 lowpriorityscore=0 mlxscore=0 impostorscore=0 mlxlogscore=999 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1810050000 definitions=main-1904030118 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: X-Virus-Scanned: ClamAV using ClamSMTP From: Julian Stecklina Only the xpfo_kunmap call that needs to actually unmap the page needs to be serialized. We need to be careful to handle the case, where after the atomic decrement of the mapcount, a xpfo_kmap increased the mapcount again. In this case, we can safely skip modifying the page table. Model-checked with up to 4 concurrent callers with Spin. Signed-off-by: Julian Stecklina Signed-off-by: Khalid Aziz Cc: Khalid Aziz Cc: x86@kernel.org Cc: kernel-hardening@lists.openwall.com Cc: Vasileios P. Kemerlis Cc: Juerg Haefliger Cc: Tycho Andersen Cc: Marco Benatto Cc: David Woodhouse --- include/linux/xpfo.h | 24 +++++++++++++++--------- 1 file changed, 15 insertions(+), 9 deletions(-) diff --git a/include/linux/xpfo.h b/include/linux/xpfo.h index 2318c7eb5fb7..37e7f52fa6ce 100644 --- a/include/linux/xpfo.h +++ b/include/linux/xpfo.h @@ -61,6 +61,7 @@ static inline void xpfo_kmap(void *kaddr, struct page *page) static inline void xpfo_kunmap(void *kaddr, struct page *page) { unsigned long flags; + bool flush_tlb = false; if (!static_branch_unlikely(&xpfo_inited)) return; @@ -72,18 +73,23 @@ static inline void xpfo_kunmap(void *kaddr, struct page *page) * The page is to be allocated back to user space, so unmap it from * the kernel, flush the TLB and tag it as a user page. */ - spin_lock_irqsave(&page->xpfo_lock, flags); - if (atomic_dec_return(&page->xpfo_mapcount) == 0) { -#ifdef CONFIG_XPFO_DEBUG - WARN_ON(PageXpfoUnmapped(page)); -#endif - SetPageXpfoUnmapped(page); - set_kpte(kaddr, page, __pgprot(0)); - xpfo_flush_kernel_tlb(page, 0); + spin_lock_irqsave(&page->xpfo_lock, flags); + + /* + * In the case, where we raced with kmap after the + * atomic_dec_return, we must not nuke the mapping. + */ + if (atomic_read(&page->xpfo_mapcount) == 0) { + SetPageXpfoUnmapped(page); + set_kpte(kaddr, page, __pgprot(0)); + flush_tlb = true; + } + spin_unlock_irqrestore(&page->xpfo_lock, flags); } - spin_unlock_irqrestore(&page->xpfo_lock, flags); + if (flush_tlb) + xpfo_flush_kernel_tlb(page, 0); } void xpfo_alloc_pages(struct page *page, int order, gfp_t gfp, bool will_map);