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b=EbDwYwoNKUocRBBgSecGIn8A3tOfN/0arc68+tvaKGxfyydiSqxN7QJUAT9euMZ67 Mk8bAHSjG0AEOv6I3Nd4BfrojmDZ7ckWlP0nA0/7747hAOkOmTc/TtrAUEpFtt3ht6 957spyvWyAa7tCz04lf/gLEIJo6Hev3f5wjiUxJg= Date: Tue, 30 Jul 2024 13:31:11 -0700 From: Andrew Morton To: Danilo Krummrich Cc: cl@linux.com, penberg@kernel.org, rientjes@google.com, iamjoonsoo.kim@lge.com, vbabka@suse.cz, roman.gushchin@linux.dev, 42.hyeyoo@gmail.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH 1/2] mm: krealloc: consider spare memory for __GFP_ZERO Message-Id: <20240730133111.d180e1a6fc63b2883fe99821@linux-foundation.org> In-Reply-To: <20240730194214.31483-1-dakr@kernel.org> References: <20240730194214.31483-1-dakr@kernel.org> X-Mailer: Sylpheed 3.8.0beta1 (GTK+ 2.24.33; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-Rspam-User: X-Stat-Signature: 5dn81tbfwocu98159scyxqq8q7maswgm X-Rspamd-Queue-Id: 03603C0006 X-Rspamd-Server: rspam11 X-HE-Tag: 1722371473-753078 X-HE-Meta: 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 4E1gzpcQ sPjAepmGQ9TbP3secZcCaAej7UfeHQCx0TNjYQv/m1ijfN9/YEJsX/TsBhNSv78fIMAdQbnm5RmxKvkw600SlMZNosH3tW4UYN1NYQSLekKHNAUAzFZeOEwQAfr4eSR/2rkLlxB/c8ciY+Oky/D8kDEOGqUBFaxEEYlypWi6j0Hqr+GUH4uNCWp/lIUppGEg/XtOgSArrTScRuQNXcL4HDnA4bDleh+1m71k2Cw6CYznWwhVYjXX5lsHygwtRDYHo5g+7+BdeKqR9QtVrJedgKqR3VNy/CcjbIVakT1Wl8VPyxSqkucNsqSGljWis/7tNd+fxm7wn6/Xd0pzcv23mhKT7ALMw3WTZsqC6f8L0eJQP6NFuG1WG2lI//f4N7lk1724OrrsjezhcmnQMTw9glb97YA== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000002, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Tue, 30 Jul 2024 21:42:05 +0200 Danilo Krummrich wrote: > As long as krealloc() is called with __GFP_ZERO consistently, starting > with the initial memory allocation, __GFP_ZERO should be fully honored. > > However, if for an existing allocation krealloc() is called with a > decreased size, it is not ensured that the spare portion the allocation > is zeroed. Thus, if krealloc() is subsequently called with a larger size > again, __GFP_ZERO can't be fully honored, since we don't know the > previous size, but only the bucket size. Well that's bad. > Example: > > buf = kzalloc(64, GFP_KERNEL); If this was kmalloc() > memset(buf, 0xff, 64); > > buf = krealloc(buf, 48, GFP_KERNEL | __GFP_ZERO); > > /* After this call the last 16 bytes are still 0xff. */ > buf = krealloc(buf, 64, GFP_KERNEL | __GFP_ZERO); then this would expose uninitialized kernel memory to kernel code, with a risk that the kernel code will expose that to userspace, yes? This does seem rather a trap, and I wonder whether krealloc() should just zero out any such data by default. > Fix this, by explicitly setting spare memory to zero, when shrinking an > allocation with __GFP_ZERO flag set or init_on_alloc enabled. > > --- a/mm/slab_common.c > +++ b/mm/slab_common.c > @@ -1273,6 +1273,13 @@ __do_krealloc(const void *p, size_t new_size, gfp_t flags) > > /* If the object still fits, repoison it precisely. */ > if (ks >= new_size) { > + /* Zero out spare memory. */ > + if (want_init_on_alloc(flags)) { > + kasan_disable_current(); > + memset((void *)p + new_size, 0, ks - new_size); Casting away the constness of `*p'. This is just misleading everyone, really. It would be better to make argument `p' have type "void *". > + kasan_enable_current(); > + } > + > p = kasan_krealloc((void *)p, new_size, flags);