Messages in this thread | | | From | Nadav Amit <> | Subject | Re: [PATCH v4 05/10] x86/alternative: initializing temporary mm for patching | Date | Sun, 11 Nov 2018 20:38:53 +0000 |
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From: Peter Zijlstra Sent: November 11, 2018 at 2:43:27 PM GMT > To: Nadav Amit <namit@vmware.com> > Cc: Ingo Molnar <mingo@redhat.com>, linux-kernel@vger.kernel.org, x86@kernel.org, H. Peter Anvin <hpa@zytor.com>, Thomas Gleixner <tglx@linutronix.de>, Borislav Petkov <bp@alien8.de>, Dave Hansen <dave.hansen@linux.intel.com>, Kees Cook <keescook@chromium.org>, Dave Hansen <dave.hansen@intel.com> > Subject: Re: [PATCH v4 05/10] x86/alternative: initializing temporary mm for patching > > > > I don't seem to have gotten patches 0-2,7 for some reason; I'll try and > dig them out of the LKML folder. > > On Sat, Nov 10, 2018 at 03:17:27PM -0800, Nadav Amit wrote: >> +void __init poking_init(void) >> +{ >> + spinlock_t *ptl; >> + pte_t *ptep; >> + >> + poking_mm = copy_init_mm(); >> + if (!poking_mm) { >> + pr_err("x86/mm: error setting a separate poking address space"); >> + return; >> + } >> + >> + /* >> + * Randomize the poking address, but make sure that the following page >> + * will be mapped at the same PMD. We need 2 pages, so find space for 3, >> + * and adjust the address if the PMD ends after the first one. >> + */ >> + poking_addr = TASK_UNMAPPED_BASE + >> + (kaslr_get_random_long("Poking") & PAGE_MASK) % >> + (TASK_SIZE - TASK_UNMAPPED_BASE - 3 * PAGE_SIZE); >> + >> + if (((poking_addr + PAGE_SIZE) & ~PMD_MASK) == 0) >> + poking_addr += PAGE_SIZE; >> + >> + /* >> + * We need to trigger the allocation of the page-tables that will be >> + * needed for poking now. Later, poking may be performed in an atomic >> + * section, which might cause allocation to fail. >> + */ >> + ptep = get_locked_pte(poking_mm, poking_addr, &ptl); >> + if (!WARN_ON(!ptep)) >> + pte_unmap_unlock(ptep, ptl); >> +} > > The difference in how we deal with -ENOMEM here is weird. I think we > have a _lot_ of code that simply hard assumes we don't fail memory alloc > on init. > > I for instance would not mind to simply remove both branches and let the > kernel crash and burn if we ever fail here.
Actually, now that we removed the fallback of patching without poking_mm, a failure to allocate poking_mm should have had a BUG_ON().
For the second case, I think we still need either WARN_ON() or BUG_ON(), at least as some sort of an in-code comment. I’ll change it to BUG_ON() if you prefer.
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