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[109.81.82.61]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-4872459d409sm19992757f8f.33.2026.09.21.00.49.35 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 21 Sep 2026 00:49:36 -0700 (PDT) Date: Mon, 21 Sep 2026 09:49:19 +0200 From: Michal Hocko To: Mohammed EL Kadiri Cc: Andrew Morton , Vlastimil Babka , Suren Baghdasaryan , Brendan Jackman , Johannes Weiner , Zi Yan , linux-mm@kvack.org, linux-kernel@vger.kernel.org, syzbot+c61d6962d0b7e698439e@syzkaller.appspotmail.com Subject: Re: [PATCH 1/1] mm/page_alloc: enforce upper bound on min_free_kbytes sysctl write Message-ID: References: <3c1598eaff225a155cdec5e92d5c97fbfc6c00fd.1789724312.git.med08elkadiri@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <3c1598eaff225a155cdec5e92d5c97fbfc6c00fd.1789724312.git.med08elkadiri@gmail.com> X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: 0975C40008 X-Rspam-User: X-Stat-Signature: et7g58gx8qzi6gxmptchz6iaqbxifc67 X-HE-Tag: 1789976977-106971 X-HE-Meta: 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 NzffcmxR 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 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Fri 18-09-26 11:45:51, Mohammed EL Kadiri wrote: > syzbot reports a kernel panic ("System is deadlocked on memory") > after writing a large value (e.g. 3145728, 3GB) to > /proc/sys/vm/min_free_kbytes. > > calculate_min_free_kbytes() already caps the auto-tuned default at > 256MB, with a comment noting larger values don't make sense even on > big machines. But that cap only applies to the computed default -- > min_free_kbytes_sysctl_handler() lets a user write any value, since > it only sets extra1 = SYSCTL_ZERO, with no extra2. > > An unbounded min_free_kbytes drives watermarks past what the system > can provide, so every allocation triggers reclaim/OOM with no way to > make progress, and out_of_memory() eventually panics. This is an admin only interface and we assume whoever has access can realize consequences of the set value. Auto-tuning really cannot make any judgment call so it needs a cap. The same doens't hold for the admin. > Fix this by applying the same 256MB cap to the sysctl write path. > > Tested on top of upstream master (5dd1818b15d9): > - Before: syzbot's reproducer reliably panics the kernel. > - After: writing 3145728 fails with -EINVAL, normal values > (e.g. 200000) still work, and the reproducer no longer panics > the kernel across repeated runs. > > Note: this covers the reported case (a value far above any real > machine's RAM). It doesn't prevent a smaller value that's still a > large fraction of a very small system's memory from causing the > same problem via lowmem_pages in __setup_per_zone_wmarks(). Open to > a follow-up there if maintainers think it's worth it. > > Reported-by: syzbot+c61d6962d0b7e698439e@syzkaller.appspotmail.com > Closes: https://syzkaller.appspot.com/bug?extid=c61d6962d0b7e698439e > Signed-off-by: Mohammed EL Kadiri There is nothing really to fix. Nacked-by: Michal Hocko > --- > mm/page_alloc.c | 4 ++++ > 1 file changed, 4 insertions(+) > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c > index 12fac9084c48..8cb62879bf41 100644 > --- a/mm/page_alloc.c > +++ b/mm/page_alloc.c > @@ -6933,6 +6933,9 @@ static int percpu_pagelist_high_fraction_sysctl_handler(const struct ctl_table * > return ret; > } > > +/* Cap sysctl writes to the same bound calculate_min_free_kbytes() uses. */ > +static const int max_user_min_free_kbytes = 262144; > + > static const struct ctl_table page_alloc_sysctl_table[] = { > { > .procname = "min_free_kbytes", > @@ -6941,6 +6944,7 @@ static const struct ctl_table page_alloc_sysctl_table[] = { > .mode = 0644, > .proc_handler = min_free_kbytes_sysctl_handler, > .extra1 = SYSCTL_ZERO, > + .extra2 = (void *)&max_user_min_free_kbytes, > }, > { > .procname = "watermark_boost_factor", > -- > 2.53.0 -- Michal Hocko SUSE Labs