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Howlett" , Alice Ryhl , rust-for-linux@vger.kernel.org Subject: Re: [PATCH] slub: fix data loss and overflow in krealloc() Message-ID: References: <20260416132837.3787694-1-elver@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspamd-Queue-Id: 3DA874000B X-Rspamd-Server: rspam12 X-Stat-Signature: he53tw7p989m696nicgyquifoc7czoxr X-Rspam-User: X-HE-Tag: 1776417700-449880 X-HE-Meta: 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 /3rWpbdb cCDpQPV5sWtZy7ee+3P+GAt7KI9b9I0gULrXFPsboi93J+lDews1E+ubT9sm6MKwsLNYgTgbHCgzefOnZ/zzTi/YhLvXXxxpBlP+vs3yHVNpzvzAfrCAPwYIIp8r/JYHNg8YVT/tjuBcT4LBXiR3wBiZ3soSsOifSWtgHL+7dPO6OZmigx+jb9zxHG6bbmKewzYpGc7bwjk4fNDmQagfB8srUMmd8qqYllak5Lwy4ntnJVTwF5h27XubEtlrra0IggWh7wnjWxsRGXIeq5Y+UsP/p5vMXAKIHk3zlfZ87aDmRueaBOnb9vsWpgZEpzou9aQBE0Y261IYXqI/3Q6MsCHzT0c3SRajGtw9jn0/ypbE0F0M= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Fri, Apr 17, 2026 at 11:05:09AM +0200, Marco Elver wrote: > On Fri, 17 Apr 2026 at 06:42, Harry Yoo (Oracle) wrote: > > [+Cc relevant folks] > > > > On Thu, Apr 16, 2026 at 03:25:07PM +0200, Marco Elver wrote: > > > Commit 2cd8231796b5 ("mm/slub: allow to set node and align in > > > k[v]realloc") introduced the ability to force a reallocation if the > > > original object does not satisfy new alignment or NUMA node, even when > > > the object is being shrunk. > > > > > > This introduced two bugs in the reallocation fallback path: > > > > > > 1. Data loss during NUMA migration: The jump to 'alloc_new' happens > > > before 'ks' and 'orig_size' are initialized. As a result, the > > > memcpy() in the 'alloc_new' block would copy 0 bytes into the new > > > allocation. > > > > Ouch. > > > > > 2. Buffer overflow during shrinking: When shrinking an object while > > > forcing a new alignment, 'new_size' is smaller than the old size. > > > However, the memcpy() used the old size ('orig_size ?: ks'), leading > > > to an out-of-bounds write. > > > > Right. before the commit we didn't reallocate when the size is smaller. > > > > > The same overflow bug exists in the kvrealloc() fallback path, where the > > > old bucket size ksize(p) is copied into the new buffer without being > > > bounded by the new size. > > > > > > A simple reproducer: > > > > > > // e.g. add to lkdtm as KREALLOC_SHRINK_OVERFLOW > > > while (1) { > > > void *p = kmalloc(128, GFP_KERNEL); > > > p = krealloc_node_align(p, 64, 256, GFP_KERNEL, NUMA_NO_NODE); > > > kfree(p); > > > } > > > > > > demonstrates the issue: > > > > > > ================================================================== > > > BUG: KFENCE: out-of-bounds write in memcpy_orig+0x68/0x130 > > > > > > Out-of-bounds write at 0xffff8883ad757038 (120B right of kfence-#47): > > > memcpy_orig+0x68/0x130 > > > krealloc_node_align_noprof+0x1c8/0x340 > > > lkdtm_KREALLOC_SHRINK_OVERFLOW+0x8c/0xc0 [lkdtm] > > > lkdtm_do_action+0x3a/0x60 [lkdtm] > > > ... > > > > > > kfence-#47: 0xffff8883ad756fc0-0xffff8883ad756fff, size=64, cache=kmalloc-64 > > > > > > allocated by task 316 on cpu 7 at 97.680481s (0.021813s ago): > > > krealloc_node_align_noprof+0x19c/0x340 > > > lkdtm_KREALLOC_SHRINK_OVERFLOW+0x8c/0xc0 [lkdtm] > > > lkdtm_do_action+0x3a/0x60 [lkdtm] > > > ... > > > ================================================================== > > > > > > Fix it by moving the old size calculation to the top of __do_krealloc() > > > and bounding all copy lengths by the new allocation size. > > > > > > Fixes: 2cd8231796b5 ("mm/slub: allow to set node and align in k[v]realloc") > > > Cc: > > > Reported-by: https://sashiko.dev/#/patchset/20260415143735.2974230-1-elver%40google.com > > > Signed-off-by: Marco Elver > > > --- > > > > Looks good to me, but I think we still have a similar issue in > > vrealloc_node_align_noprof()? (goto need_realloc; due to NUMA mismatch > > but the new size is smaller) > > Good find. > > That's a separate patch, though, since it's in the vmalloc subsystem You're right. for this patch: Looks good to me, Reviewed-by: Harry Yoo (Oracle) > (it's also not confidence-inspiring that vrealloc_node_align_noprof > has a bunch of TODOs sprinkled all over...). ;) looks like one of the TODOs is going to be tacked though. (shrinking) https://lore.kernel.org/linux-mm/20260404-vmalloc-shrink-v10-0-335759165dfa@zohomail.in > Since you found that, do you want to claim it? I have many stuffs going on my plate now (including re-reviewing the typed kmalloc caches patch) so it'd be nice if somebody could claim :) > Also by the looks of it slub and vmalloc patches go through different > trees these days per MAINTAINERS. Right. slab patches go through the slab tree. -- Cheers, Harry / Hyeonggon