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Shutemov" , Nic Losby , Yisheng Xie , stable@vger.kernel.org Subject: Re: [PATCH v2] hugetlbfs: check for pgoff value overflow Message-Id: <20180308141533.d16e43f5f559215089e522ae@linux-foundation.org> In-Reply-To: <20180308210502.15952-1-mike.kravetz@oracle.com> References: <20180306133135.4dc344e478d98f0e29f47698@linux-foundation.org> <20180308210502.15952-1-mike.kravetz@oracle.com> X-Mailer: Sylpheed 3.5.1 (GTK+ 2.24.31; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: stable-owner@vger.kernel.org X-Mailing-List: stable@vger.kernel.org X-getmail-retrieved-from-mailbox: INBOX X-Mailing-List: linux-kernel@vger.kernel.org List-ID: On Thu, 8 Mar 2018 13:05:02 -0800 Mike Kravetz wrote: > A vma with vm_pgoff large enough to overflow a loff_t type when > converted to a byte offset can be passed via the remap_file_pages > system call. The hugetlbfs mmap routine uses the byte offset to > calculate reservations and file size. > > A sequence such as: > mmap(0x20a00000, 0x600000, 0, 0x66033, -1, 0); > remap_file_pages(0x20a00000, 0x600000, 0, 0x20000000000000, 0); > will result in the following when task exits/file closed, > kernel BUG at mm/hugetlb.c:749! > Call Trace: > hugetlbfs_evict_inode+0x2f/0x40 > evict+0xcb/0x190 > __dentry_kill+0xcb/0x150 > __fput+0x164/0x1e0 > task_work_run+0x84/0xa0 > exit_to_usermode_loop+0x7d/0x80 > do_syscall_64+0x18b/0x190 > entry_SYSCALL_64_after_hwframe+0x3d/0xa2 > > The overflowed pgoff value causes hugetlbfs to try to set up a > mapping with a negative range (end < start) that leaves invalid > state which causes the BUG. > > The previous overflow fix to this code was incomplete and did not > take the remap_file_pages system call into account. > > --- a/fs/hugetlbfs/inode.c > +++ b/fs/hugetlbfs/inode.c > @@ -111,6 +111,7 @@ static void huge_pagevec_release(struct pagevec *pvec) > static int hugetlbfs_file_mmap(struct file *file, struct vm_area_struct *vma) > { > struct inode *inode = file_inode(file); > + unsigned long ovfl_mask; > loff_t len, vma_len; > int ret; > struct hstate *h = hstate_file(file); > @@ -127,12 +128,16 @@ static int hugetlbfs_file_mmap(struct file *file, struct vm_area_struct *vma) > vma->vm_ops = &hugetlb_vm_ops; > > /* > - * Offset passed to mmap (before page shift) could have been > - * negative when represented as a (l)off_t. > + * page based offset in vm_pgoff could be sufficiently large to > + * overflow a (l)off_t when converted to byte offset. > */ > - if (((loff_t)vma->vm_pgoff << PAGE_SHIFT) < 0) > + ovfl_mask = (1UL << (PAGE_SHIFT + 1)) - 1; > + ovfl_mask <<= ((sizeof(unsigned long) * BITS_PER_BYTE) - > + (PAGE_SHIFT + 1)); That's a compile-time constant. The compiler will indeed generate an immediate load, but I think it would be better to make the code look more like we know that it's a constant, if you get what I mean. Something like /* * If a pgoff_t is to be converted to a byte index, this is the max value it * can have to avoid overflow in that conversion. */ #define PGOFF_T_MAX And I bet that this constant could be used elsewhere - surely it's a very common thing to be checking for. Also, the expression seems rather complicated. Why are we adding 1 to PAGE_SHIFT? Isn't there a logical way of using PAGE_MASK? The resulting constant is 0xfff8000000000000 on 64-bit. We could just use along the lines of 1UL << (BITS_PER_LONG - PAGE_SHIFT - 1) But why the -1? We should be able to handle a pgoff_t of 0xfff0000000000000 in this code? Also, we later to len = vma_len + ((loff_t)vma->vm_pgoff << PAGE_SHIFT); /* check for overflow */ if (len < vma_len) return -EINVAL; which is ungainly: even if we passed the PGOFF_T_MAX test, there can still be an overflow which we still must check for. Is that avoidable? Probably not...