linux-kernel - Re: [PATCH] x86_64: fix delayed signals

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <alpine.LFD.1.10.0807131121430.2959@woody.linux-foundation.org>
Date:	Sun, 13 Jul 2008 11:36:02 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Oleg Nesterov <oleg@...sign.ru>
cc:	Denys Vlasenko <vda.linux@...glemail.com>,
	T?r?k Edwin <edwintorok@...il.com>,
	Ingo Molnar <mingo@...e.hu>,
	Roland McGrath <roland@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Elias Oltmanns <eo@...ensachen.de>,
	Arjan van de Ven <arjan@...radead.org>
Subject: Re: [PATCH] x86_64: fix delayed signals

On Sun, 13 Jul 2008, Oleg Nesterov wrote:
> 
> SETFL_MASK doesn't have O_RDWR, and in any case setfl() changes ->f_flags,
> not ->f_mode.

I think you can still read a write-only file descriptor on some systems: 
doing a mmap( PROT_WRITE ) on it can sometimes do it.

The reason - PROT_WRITE often implies PROT_READ (because on many CPU's you 
have to do unaligned writes with a read-modify-write cycle - on some like 
old alphas you have to do even normal byte and word writes that way).

So if the OS checks just the asked-for protections against the file 
descriptor protections, you'll essentially get read access for free when 
you do the mmap.

At least current versions of Linux won't ever allow a file mapping of a 
non-readable fd, but I won't guarantee we always did that, nor that other 
OS's always do it.

Anyway, I misunderstood Edwins idea to actually throw away the writes if 
the file isn't linked anywhere and is only open for writing. I guess we 
could optimize that, but it's such an unrealistic specialc case that it 
really isn't worth it except for some benchmark-gaming kind of thing 
(which I don't like doing anyway).

				Linus
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/