From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-pg1-f176.google.com (mail-pg1-f176.google.com [209.85.215.176]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 364EB34C134 for ; Wed, 12 Aug 2026 07:27:07 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.215.176 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1786519628; cv=none; b=G4QXq7EFzGjZ2T4w2mSWSDWDyzwUAriiBHQWSJNAT3rMm1y4hL6n95bRNzY2vwypFYhOZMosqL4bAYFiVxTDgqcbrEVInwsszjU59l1mlAC4uAgbmkgr3ZGCu3xzl9Us+EgwqPmsMDHHG2k0wnv3rRghc4ljJHOZEycqTAcl9P0= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1786519628; c=relaxed/simple; bh=9+H/1bC9AngwY48oSSf37D0mjWXZXMcBbkIDQidIu/k=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=UI3pfhNYcgHfj3kHvV/+ruRdnb01Im3ZZeIAmuylxdPuwNB9fEPMimBopiGFc3QzwZWPixUtaMX3DkMHUMpgC1KjETdwX7tCfRadXT+cYOwuNrdGjy9c/1NF0iPoCASPmy5DqFhMskAvz01nHoS8a2HWr85CnXWdkD8GfBKIBgU= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=gmail.com; spf=pass smtp.mailfrom=gmail.com; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b=BwvgCTXW; arc=none smtp.client-ip=209.85.215.176 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=gmail.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=gmail.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b="BwvgCTXW" Received: by mail-pg1-f176.google.com with SMTP id 41be03b00d2f7-c9eefcf9175so565099a12.3 for ; Wed, 12 Aug 2026 00:27:07 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20251104; t=1786519626; x=1787124426; darn=vger.kernel.org; h=in-reply-to:content-disposition:content-type:mime-version :references:message-id:subject:cc:to:from:date:from:to:cc:subject :date:message-id:reply-to:content-type; bh=3H2XRYBLrEGNhf/P+5Th74cqCMRbv9XKacb8u8bUl28=; b=BwvgCTXWipJfuECtyiYCDBxDvDlUrPx98tO5SQOyml9oqh7v0u9LFjahfOzNxAeQmv GoXznTxZ4RFVZUJO2vi3R3nym2jXSiWNyR+Symfug3PeeObhUMrHYLnaBVH7TCN8Y85w x4i9lbUNPs/6qfEkp+jFDuMGBUpgSrx5XStXh+XLGfcqcQ5EUuvxkgMANFYOOCvC8cf1 JFvYKSqnh2Z91mDJE4swyZFz5MGsrkah9XUjs6BqNtGNc+7jnWP5kyuprN74miZyAj1B lATg5KqHNVUJZ6LduRXNkP34UFQLrMfdFreQqMLWrcGFSjK9hwt0wI1QC4+O8+Mm4p3X KOLQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20251104; t=1786519626; x=1787124426; h=in-reply-to:content-disposition:content-type:mime-version :references:message-id:subject:cc:to:from:date:x-gm-gg :x-gm-message-state:from:to:cc:subject:date:message-id:reply-to :content-type; bh=3H2XRYBLrEGNhf/P+5Th74cqCMRbv9XKacb8u8bUl28=; b=cEfyeD5gR5NK4DpgNd24UXN/bAasGiuMJ6JaeX6VualHrIXlNstSJCplknKhgQzXXl gUrRlaeUqonHFR0IjSp6Ceo13vzWWK/Jnb5fvJ5xd1TT1xIyLFVarvHtLe2qgdUMje/T 0SQWS01x7KBr0h/1i/VGj24iLvzInN0j5V5eRmcjSdz1ajk7m42I+batUg3ig1x9EizW 3rXapD8RS0nA8/zVEhBegLltNm1sQVLbE9jQn+H6aRwO0YsqGoRJb8CaAG+Nx/orJeIN 51JDEesVnWzZD1yurvgWEYDJqOnlBYBuVlAMijuuJEmHt8xhY5xqozYJQLPusJMwhPII BKwg== X-Forwarded-Encrypted: i=1; AHgh+RqbSrvIv4a7Bwbe/jD+1NrU4P/t08PCauUU4c+0hiCJI4Oi1AfcHqRyOwp4IOaonbTTEBhGOuzVubs=@vger.kernel.org X-Gm-Message-State: AOJu0Yxd+ORIipbtTBGm9fZxsEe0UvHmx9O9wMIf+J5l/30UQ6Els2kT gLhwoMia/p8gC/DzuwXZ/24NwpGp2TBKb+v/dQdZdyxqvZs6DoZ9+6xG X-Gm-Gg: AR+sD13uw0iiMuZmP+/xFWnUYj/1aj8olCFmxqhPstdUN9kZ1UjrTLvqXzoQi3ZioYM wHPG7Nv5BWGJrtHuJnBlB8HB5ROd/FPLPRGKydjHoJtQcbfT1qMCAXo/fakPZMKz7p0hRCe+W5e KSEsTCw50blPsPlrLDB78ah2WjkyxwYgXqwi9iyIbuzry1oANBrzCKJCNixzPjgIyCqjKggvJDS 2hvELNyDeBp5N4WQ/NL9w5O27RnK+P+K/lxegD1DDTS7SpS7TQsD3t9ukHbhY2J2eGq+Dponyve CHR3iL8fZCLHDG2EM/icV6NMiyjLaXTKfCtAgU8/T40xt+BzlnFZcwPvk50ZSlzLKQaRZ8/eq+Z 8dgpOhC3FrLUVHko1Ss6qNM0n/ho0d/EQ/bjeuGSQluHLpkvNQ54YbIs89E6qNqaLKG0fM7BGHL 3rAZ8NVRreEAGwibLGBU/xkJftp6JPh4mx4sJL1Ag7BJDT/XQnt971GzoaxdWmsqFO8behByZqc yeMqg== X-Received: by 2002:a05:6a21:329a:b0:3c3:bbe6:95c7 with SMTP id adf61e73a8af0-3cc3f701ebbmr2837875637.17.1786519626456; Wed, 12 Aug 2026 00:27:06 -0700 (PDT) Received: from kernel ([45.251.35.26]) by smtp.gmail.com with ESMTPSA id 5a478bee46e88-31cf6d79bd6sm8297129eec.28.2026.08.12.00.27.03 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 12 Aug 2026 00:27:05 -0700 (PDT) Date: Wed, 12 Aug 2026 12:56:59 +0530 From: Mohamad Raizudeen To: Greg KH Cc: bhelgaas@google.com, skhan@linuxfoundation.org, jkoolstra@xs4all.nl, linux-pci@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] PCI: Fix use-after-free race in pci_find_bus() Message-ID: References: <20260812024713.4958-1-raizudeen.kerneldev@gmail.com> <2026081227-sandy-imaginary-fa49@gregkh> Precedence: bulk X-Mailing-List: linux-pci@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: <2026081227-sandy-imaginary-fa49@gregkh> On Wed, Aug 12, 2026 at 12:00:33PM +0900, Greg KH wrote: > On Wed, Aug 12, 2026 at 08:17:13AM +0530, Mohamad Raizudeen wrote: > > pci_find_bus() iterates over the list of PCI root buses using > > pci_find_next_bus(). This helper acquires pci_bus_sem, retrieves the > > next bus and drops the lock before returning the pointer to the caller. > > > > pci_find_bus() then uses this pointer to check the domain and traverses > > the child buses via pci_do_find_bus() without holding the pci_bus_sem > > lock. > > > > If a PCI bus is concurrently removed for example via hotplug between > > loop iterations, the from pointer passed back into pci_find_next_bus() > > becomes stale, leading to a user-after-free when dereferencing > > from->node.next. Additionally, traversing the bus tree without holding > > the lock is a race condition. > > Did you find this actually happens? How did you find this at all? I found this purely through by reading and reviewing the code, while analyzing the locking patterns in the PCI subsystem. I have not seen it crash in production, but the race condition is statically clear from reading the code. > > > > > Fix this by iterating pci_root_buses list directly using > > list_for_each_entry() inside pci_find_bus() while holding the > > pci_bus_sem read lock for the entire duration of the search. This > > ensures the list and tree structures cannot change while being > > traversed, eliminating the use-after-free. > > Why do two changes here, and not just make a patch series? I did both in one patch because they are connected. Since pci_find_next_bus() drops the lock early, I couldn't use it to hold the lock for the whole search. I had to change the loop to fix the locking. > > > Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2") > > Signed-off-by: Mohamad Raizudeen > > --- > > drivers/pci/search.c | 20 +++++++++++--------- > > 1 file changed, 11 insertions(+), 9 deletions(-) > > > > diff --git a/drivers/pci/search.c b/drivers/pci/search.c > > index e3d3177fce54..f50e83061b76 100644 > > --- a/drivers/pci/search.c > > +++ b/drivers/pci/search.c > > @@ -142,17 +142,19 @@ static struct pci_bus *pci_do_find_bus(struct pci_bus *bus, unsigned char busnr) > > */ > > struct pci_bus *pci_find_bus(int domain, int busnr) > > { > > - struct pci_bus *bus = NULL; > > - struct pci_bus *tmp_bus; > > + struct pci_bus *bus; > > + struct pci_bus *tmp_bus = NULL; > > > > - while ((bus = pci_find_next_bus(bus)) != NULL) { > > - if (pci_domain_nr(bus) != domain) > > - continue; > > - tmp_bus = pci_do_find_bus(bus, busnr); > > - if (tmp_bus) > > - return tmp_bus; > > + down_read(&pci_bus_sem); > > + list_for_each_entry(bus, &pci_root_buses, node) { > > + if (pci_domain_nr(bus) == domain) { > > + tmp_bus = pci_do_find_bus(bus, busnr); > > + if (tmp_bus) > > + break; > > + } > > Are you sure this logic is the same as the original? Yes, I used list_for_each_entry() that does the exact same thing as the old while loop, but it let me keep the lock saefely for the whole search. > pci_find_next_bus() does grab the needed lock here, so why do you think > this is racy? You are right, it grabs the lock. But it drops the lock before returning the bus pointer. So the caller then uses that pointer without a lock and passes it back for the next loop. If a bus is removed in that moment, the next call reads freed memory. > > And pci_bus_sem is just for root busses, not the individual busses, > right? It protects the root bus list, but it also protects the child buses. Since pci_do_find_bus() walks through the child buses, needed to hold the lock to make that safe too. > > How was this tested? I compiled ir and booted it in x86_64 qemu vm. It booted fine without any PCI crashes. I will run the same test on v2 patch before sending it. > > > } > > - return NULL; > > + up_read(&pci_bus_sem); > > Why not use guard() instead? Honestly, I was so focused on getting the locking logic right that I completely missed I meant forgot to use guard(). I will make sure to use guard() in the v2 patch. > > thanks, > > greg k-h Thanks & regards, Mohamad Raizudeen