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[86.164.109.77]) by smtp.gmail.com with ESMTPSA id b1-20020a05600003c100b0033d81d9c44esm5995447wrg.70.2024.02.22.10.54.12 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 22 Feb 2024 10:54:12 -0800 (PST) Date: Thu, 22 Feb 2024 18:51:58 +0000 From: Lorenzo Stoakes To: Vlastimil Babka Cc: Andrew Morton , linux-mm@kvack.org, linux-kernel@vger.kernel.org, "Liam R. Howlett" , Michal Hocko , stable@vger.kernel.org Subject: Re: [PATCH] mm, mmap: fix vma_merge() case 7 with vma_ops->close Message-ID: References: <20240222165549.32753-2-vbabka@suse.cz> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20240222165549.32753-2-vbabka@suse.cz> On Thu, Feb 22, 2024 at 05:55:50PM +0100, Vlastimil Babka wrote: > When debugging issues with a workload using SysV shmem, Michal Hocko has > come up with a reproducer that shows how a series of mprotect() > operations can result in an elevated shm_nattch and thus leak of the > resource. > > The problem is caused by wrong assumptions in vma_merge() commit > 714965ca8252 ("mm/mmap: start distinguishing if vma can be removed in > mergeability test"). The shmem vmas have a vma_ops->close callback > that decrements shm_nattch, and we remove the vma without calling it. > > vma_merge() has thus historically avoided merging vma's with > vma_ops->close and commit 714965ca8252 was supposed to keep it that way. > It relaxed the checks for vma_ops->close in can_vma_merge_after() > assuming that it is never called on a vma that would be a candidate for > removal. However, the vma_merge() code does also use the result of this > check in the decision to remove a different vma in the merge case 7. > > A robust solution would be to refactor vma_merge() code in a way that > the vma_ops->close check is only done for vma's that are actually going > to be removed, and not as part of the preliminary checks. That would > both solve the existing bug, and also allow additional merges that the > checks currently prevent unnecessarily in some cases. Let's do that pretty soon :) this is a bit of an ugly fix but understandable to do it in this form to make it easier to backport (+ perhaps generate some CVEs? :) > > However to fix the existing bug first with a minimized risk, and for > easier stable backports, this patch only adds a vma_ops->close check to > the buggy case 7 specifically. All other cases of vma removal are > covered by the can_vma_merge_before() check that includes the test for > vma_ops->close. I concur, all the other cases require merge_next which would have invoked can_vma_merge_before() that calls is_mergeable_vma() with may_remove_vma set to true hence performs the close check. > > The reproducer code, adapted from Michal Hocko's code: > > int main(int argc, char *argv[]) { > int segment_id; > size_t segment_size = 20 * PAGE_SIZE; > char * sh_mem; > struct shmid_ds shmid_ds; > > key_t key = 0x1234; > segment_id = shmget(key, segment_size, > IPC_CREAT | IPC_EXCL | S_IRUSR | S_IWUSR); > sh_mem = (char *)shmat(segment_id, NULL, 0); > > mprotect(sh_mem + 2*PAGE_SIZE, PAGE_SIZE, PROT_NONE); > > mprotect(sh_mem + PAGE_SIZE, PAGE_SIZE, PROT_WRITE); > > mprotect(sh_mem + 2*PAGE_SIZE, PAGE_SIZE, PROT_WRITE); > > shmdt(sh_mem); > > shmctl(segment_id, IPC_STAT, &shmid_ds); > printf("nattch after shmdt(): %lu (expected: 0)\n", shmid_ds.shm_nattch); > > if (shmctl(segment_id, IPC_RMID, 0)) > printf("IPCRM failed %d\n", errno); > return (shmid_ds.shm_nattch) ? 1 : 0; > } > > Fixes: 714965ca8252 ("mm/mmap: start distinguishing if vma can be removed in mergeability test") > Reported-by: Michal Hocko > Cc: > Signed-off-by: Vlastimil Babka > --- > mm/mmap.c | 11 ++++++++++- > 1 file changed, 10 insertions(+), 1 deletion(-) > > diff --git a/mm/mmap.c b/mm/mmap.c > index d89770eaab6b..a4238373ee9b 100644 > --- a/mm/mmap.c > +++ b/mm/mmap.c > @@ -954,10 +954,19 @@ static struct vm_area_struct > } else if (merge_prev) { /* case 2 */ > if (curr) { > vma_start_write(curr); > - err = dup_anon_vma(prev, curr, &anon_dup); > if (end == curr->vm_end) { /* case 7 */ > + /* > + * can_vma_merge_after() assumed we would not be > + * removing prev vma, so it skipped the check > + * for vm_ops->close, but we are removing curr > + */ > + if (curr->vm_ops && curr->vm_ops->close) > + err = -EINVAL; > + else > + err = dup_anon_vma(prev, curr, &anon_dup); > remove = curr; > } else { /* case 5 */ > + err = dup_anon_vma(prev, curr, &anon_dup); This (ironically) duplicates code, could we pull this out of the if/else and put it afterwards like: if (!err) err = dup_anon_vma(prev, curr, &anon_dup); > adjust = curr; > adj_start = (end - curr->vm_start); > } > -- > 2.43.1 >