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Tue, 01 Aug 2023 09:39:04 -0700 (PDT) Received: from localhost ([2a00:23c5:dc8c:8701:1663:9a35:5a7b:1d76]) by smtp.gmail.com with ESMTPSA id m7-20020a05600c280700b003fe1afb99a9sm8453826wmb.11.2023.08.01.09.39.03 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 01 Aug 2023 09:39:03 -0700 (PDT) Date: Tue, 1 Aug 2023 17:39:02 +0100 From: Lorenzo Stoakes To: David Hildenbrand Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Andrew Morton , Baoquan He , Uladzislau Rezki , linux-fsdevel@vger.kernel.org, Jiri Olsa , Will Deacon , Mike Galbraith , Mark Rutland , wangkefeng.wang@huawei.com, catalin.marinas@arm.com, ardb@kernel.org, Linux regression tracking , regressions@lists.linux.dev, Matthew Wilcox , Liu Shixin , Jens Axboe , Alexander Viro , stable@vger.kernel.org Subject: Re: [PATCH] fs/proc/kcore: reinstate bounce buffer for KCORE_TEXT regions Message-ID: <525a3f14-74fa-4c22-9fca-9dab4de8a0c3@lucifer.local> References: <20230731215021.70911-1-lstoakes@gmail.com> <0af1bc20-8ba2-c6b6-64e6-c1f58d521504@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Aug 01, 2023 at 06:34:26PM +0200, David Hildenbrand wrote: > On 01.08.23 18:33, Lorenzo Stoakes wrote: > > On Tue, Aug 01, 2023 at 11:05:40AM +0200, David Hildenbrand wrote: > > > On 31.07.23 23:50, Lorenzo Stoakes wrote: > > > > Some architectures do not populate the entire range categorised by > > > > KCORE_TEXT, so we must ensure that the kernel address we read from is > > > > valid. > > > > > > > > Unfortunately there is no solution currently available to do so with a > > > > purely iterator solution so reinstate the bounce buffer in this instance so > > > > we can use copy_from_kernel_nofault() in order to avoid page faults when > > > > regions are unmapped. > > > > > > > > This change partly reverts commit 2e1c0170771e ("fs/proc/kcore: avoid > > > > bounce buffer for ktext data"), reinstating the bounce buffer, but adapts > > > > the code to continue to use an iterator. > > > > > > > > Fixes: 2e1c0170771e ("fs/proc/kcore: avoid bounce buffer for ktext data") > > > > Reported-by: Jiri Olsa > > > > Closes: https://lore.kernel.org/all/ZHc2fm+9daF6cgCE@krava > > > > Cc: stable@vger.kernel.org > > > > Signed-off-by: Lorenzo Stoakes > > > > --- > > > > fs/proc/kcore.c | 26 +++++++++++++++++++++++++- > > > > 1 file changed, 25 insertions(+), 1 deletion(-) > > > > > > > > diff --git a/fs/proc/kcore.c b/fs/proc/kcore.c > > > > index 9cb32e1a78a0..3bc689038232 100644 > > > > --- a/fs/proc/kcore.c > > > > +++ b/fs/proc/kcore.c > > > > @@ -309,6 +309,8 @@ static void append_kcore_note(char *notes, size_t *i, const char *name, > > > > static ssize_t read_kcore_iter(struct kiocb *iocb, struct iov_iter *iter) > > > > { > > > > + struct file *file = iocb->ki_filp; > > > > + char *buf = file->private_data; > > > > loff_t *fpos = &iocb->ki_pos; > > > > size_t phdrs_offset, notes_offset, data_offset; > > > > size_t page_offline_frozen = 1; > > > > @@ -554,11 +556,22 @@ static ssize_t read_kcore_iter(struct kiocb *iocb, struct iov_iter *iter) > > > > fallthrough; > > > > case KCORE_VMEMMAP: > > > > case KCORE_TEXT: > > > > + /* > > > > + * Sadly we must use a bounce buffer here to be able to > > > > + * make use of copy_from_kernel_nofault(), as these > > > > + * memory regions might not always be mapped on all > > > > + * architectures. > > > > + */ > > > > + if (copy_from_kernel_nofault(buf, (void *)start, tsz)) { > > > > + if (iov_iter_zero(tsz, iter) != tsz) { > > > > + ret = -EFAULT; > > > > + goto out; > > > > + } > > > > /* > > > > * We use _copy_to_iter() to bypass usermode hardening > > > > * which would otherwise prevent this operation. > > > > */ > > > > > > Having a comment at this indentation level looks for the else case looks > > > kind of weird. > > > > Yeah, but having it indented again would be weird and seem like it doesn't > > apply to the block below, there's really no good spot for it and > > checkpatch.pl doesn't mind so I think this is ok :) > > > > > > > > (does that comment still apply?) > > > > Hm good point, actually, now we're using the bounce buffer we don't need to > > avoid usermode hardening any more. > > > > However since we've established a bounce buffer ourselves its still > > appropriate to use _copy_to_iter() as we know the source region is good to > > copy from. > > > > To make life easy I'll just respin with an updated comment :) > > I'm not too picky this time, no need to resend if everybody else is fine :P > Haha you know the classic Lorenzo respin spiral and want to avoid it I see ;) The comment is actually inaccurate now, so to avoid noise + make life easy (maybe) for Andrew here's a fix patch that just corrects the comment:- ----8<---- >From d2b8fb271f21b79048e5630699133f77a93d0481 Mon Sep 17 00:00:00 2001 From: Lorenzo Stoakes Date: Tue, 1 Aug 2023 17:36:08 +0100 Subject: [PATCH] fs/proc/kcore: correct comment Correct comment to be strictly correct about reasoning. Signed-off-by: Lorenzo Stoakes --- fs/proc/kcore.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff --git a/fs/proc/kcore.c b/fs/proc/kcore.c index 3bc689038232..23fc24d16b31 100644 --- a/fs/proc/kcore.c +++ b/fs/proc/kcore.c @@ -568,8 +568,8 @@ static ssize_t read_kcore_iter(struct kiocb *iocb, struct iov_iter *iter) goto out; } /* - * We use _copy_to_iter() to bypass usermode hardening - * which would otherwise prevent this operation. + * We know the bounce buffer is safe to copy from, so + * use _copy_to_iter() directly. */ } else if (_copy_to_iter(buf, tsz, iter) != tsz) { ret = -EFAULT; -- 2.41.0