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Biederman" Cc: Jing Xiangfeng , kstewart@linuxfoundation.org, gregkh@linuxfoundation.org, gustavo@embeddedor.com, bhelgaas@google.com, tglx@linutronix.de, sakari.ailus@linux.intel.com, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH] arm: fix page faults in do_alignment Message-ID: <20190906151759.GM13294@shell.armlinux.org.uk> References: <1567171877-101949-1-git-send-email-jingxiangfeng@huawei.com> <20190830133522.GZ13294@shell.armlinux.org.uk> <87d0gmwi73.fsf@x220.int.ebiederm.org> <20190830203052.GG13294@shell.armlinux.org.uk> <87y2zav01z.fsf@x220.int.ebiederm.org> <20190830222906.GH13294@shell.armlinux.org.uk> <87mufmioqv.fsf@x220.int.ebiederm.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87mufmioqv.fsf@x220.int.ebiederm.org> User-Agent: Mutt/1.10.1 (2018-07-13) X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Mon, Sep 02, 2019 at 12:36:56PM -0500, Eric W. Biederman wrote: > Russell King - ARM Linux admin writes: > > > On Fri, Aug 30, 2019 at 04:02:48PM -0500, Eric W. Biederman wrote: > >> Russell King - ARM Linux admin writes: > >> > >> > On Fri, Aug 30, 2019 at 02:45:36PM -0500, Eric W. Biederman wrote: > >> >> Russell King - ARM Linux admin writes: > >> >> > >> >> > On Fri, Aug 30, 2019 at 09:31:17PM +0800, Jing Xiangfeng wrote: > >> >> >> The function do_alignment can handle misaligned address for user and > >> >> >> kernel space. If it is a userspace access, do_alignment may fail on > >> >> >> a low-memory situation, because page faults are disabled in > >> >> >> probe_kernel_address. > >> >> >> > >> >> >> Fix this by using __copy_from_user stead of probe_kernel_address. > >> >> >> > >> >> >> Fixes: b255188 ("ARM: fix scheduling while atomic warning in alignment handling code") > >> >> >> Signed-off-by: Jing Xiangfeng > >> >> > > >> >> > NAK. > >> >> > > >> >> > The "scheduling while atomic warning in alignment handling code" is > >> >> > caused by fixing up the page fault while trying to handle the > >> >> > mis-alignment fault generated from an instruction in atomic context. > >> >> > > >> >> > Your patch re-introduces that bug. > >> >> > >> >> And the patch that fixed scheduling while atomic apparently introduced a > >> >> regression. Admittedly a regression that took 6 years to track down but > >> >> still. > >> > > >> > Right, and given the number of years, we are trading one regression for > >> > a different regression. If we revert to the original code where we > >> > fix up, we will end up with people complaining about a "new" regression > >> > caused by reverting the previous fix. Follow this policy and we just > >> > end up constantly reverting the previous revert. > >> > > >> > The window is very small - the page in question will have had to have > >> > instructions read from it immediately prior to the handler being entered, > >> > and would have had to be made "old" before subsequently being unmapped. > >> > >> > Rather than excessively complicating the code and making it even more > >> > inefficient (as in your patch), we could instead retry executing the > >> > instruction when we discover that the page is unavailable, which should > >> > cause the page to be paged back in. > >> > >> My patch does not introduce any inefficiencies. It onlys moves the > >> check for user_mode up a bit. My patch did duplicate the code. > >> > >> > If the page really is unavailable, the prefetch abort should cause a > >> > SEGV to be raised, otherwise the re-execution should replace the page. > >> > > >> > The danger to that approach is we page it back in, and it gets paged > >> > back out before we're able to read the instruction indefinitely. > >> > >> I would think either a little code duplication or a function that looks > >> at user_mode(regs) and picks the appropriate kind of copy to do would be > >> the best way to go. Because what needs to happen in the two cases for > >> reading the instruction are almost completely different. > > > > That is what I mean. I'd prefer to avoid that with the large chunk of > > code. How about instead adding a local replacement for > > probe_kernel_address() that just sorts out the reading, rather than > > duplicating all the code to deal with thumb fixup. > > So something like this should be fine? > > Jing Xiangfeng can you test this please? I think this fixes your issue > but I don't currently have an arm development box where I could test this. Sorry, only just got around to this again. What I came up with is this: 8<=== From: Russell King Subject: [PATCH] ARM: mm: fix alignment Signed-off-by: Russell King --- arch/arm/mm/alignment.c | 44 ++++++++++++++++++++++++++++++++++++-------- 1 file changed, 36 insertions(+), 8 deletions(-) diff --git a/arch/arm/mm/alignment.c b/arch/arm/mm/alignment.c index 6067fa4de22b..529f54d94709 100644 --- a/arch/arm/mm/alignment.c +++ b/arch/arm/mm/alignment.c @@ -765,6 +765,36 @@ do_alignment_t32_to_handler(unsigned long *pinstr, struct pt_regs *regs, return NULL; } +static int alignment_get_arm(struct pt_regs *regs, u32 *ip, unsigned long *inst) +{ + u32 instr = 0; + int fault; + + if (user_mode(regs)) + fault = get_user(instr, ip); + else + fault = probe_kernel_address(ip, instr); + + *inst = __mem_to_opcode_arm(instr); + + return fault; +} + +static int alignment_get_thumb(struct pt_regs *regs, u16 *ip, u16 *inst) +{ + u16 instr = 0; + int fault; + + if (user_mode(regs)) + fault = get_user(instr, ip); + else + fault = probe_kernel_address(ip, instr); + + *inst = __mem_to_opcode_thumb16(instr); + + return fault; +} + static int do_alignment(unsigned long addr, unsigned int fsr, struct pt_regs *regs) { @@ -772,10 +802,10 @@ do_alignment(unsigned long addr, unsigned int fsr, struct pt_regs *regs) unsigned long instr = 0, instrptr; int (*handler)(unsigned long addr, unsigned long instr, struct pt_regs *regs); unsigned int type; - unsigned int fault; u16 tinstr = 0; int isize = 4; int thumb2_32b = 0; + int fault; if (interrupts_enabled(regs)) local_irq_enable(); @@ -784,15 +814,14 @@ do_alignment(unsigned long addr, unsigned int fsr, struct pt_regs *regs) if (thumb_mode(regs)) { u16 *ptr = (u16 *)(instrptr & ~1); - fault = probe_kernel_address(ptr, tinstr); - tinstr = __mem_to_opcode_thumb16(tinstr); + + fault = alignment_get_thumb(regs, ptr, &tinstr); if (!fault) { if (cpu_architecture() >= CPU_ARCH_ARMv7 && IS_T32(tinstr)) { /* Thumb-2 32-bit */ - u16 tinst2 = 0; - fault = probe_kernel_address(ptr + 1, tinst2); - tinst2 = __mem_to_opcode_thumb16(tinst2); + u16 tinst2; + fault = alignment_get_thumb(regs, ptr + 1, &tinst2); instr = __opcode_thumb32_compose(tinstr, tinst2); thumb2_32b = 1; } else { @@ -801,8 +830,7 @@ do_alignment(unsigned long addr, unsigned int fsr, struct pt_regs *regs) } } } else { - fault = probe_kernel_address((void *)instrptr, instr); - instr = __mem_to_opcode_arm(instr); + fault = alignment_get_arm(regs, (void *)instrptr, &instr); } if (fault) { -- 2.7.4 -- RMK's Patch system: https://www.armlinux.org.uk/developer/patches/ FTTC broadband for 0.8mile line in suburbia: sync at 12.1Mbps down 622kbps up According to speedtest.net: 11.9Mbps down 500kbps up