[ Upstream commit 0f02cfbc3d9e413d450d8d0fd660077c23f67eff ]

When a system suffers from dcache aliasing a user program may observe stale VDSO data from an aliased cache line. Notably this can break the expectation that clock_gettime(CLOCK_MONOTONIC, ...) is, as its name suggests, monotonic.

In order to ensure that users observe updates to the VDSO data page as intended, align the user mappings of the VDSO data page such that their cache colouring matches that of the virtual address range which the kernel will use to update the data page - typically its unmapped address within kseg0.

This ensures that we don't introduce aliasing cache lines for the VDSO data page, and therefore that userland will observe updates without requiring cache invalidation.

Signed-off-by: Paul Burton paul.burton@mips.com Reported-by: Hauke Mehrtens hauke@hauke-m.de Reported-by: Rene Nielsen rene.nielsen@microsemi.com Reported-by: Alexandre Belloni alexandre.belloni@bootlin.com Fixes: ebb5e78cc634 ("MIPS: Initial implementation of a VDSO") Patchwork: https://patchwork.linux-mips.org/patch/20344/ Tested-by: Alexandre Belloni alexandre.belloni@bootlin.com Tested-by: Hauke Mehrtens hauke@hauke-m.de Cc: James Hogan jhogan@kernel.org Cc: linux-mips@linux-mips.org Cc: stable@vger.kernel.org # v4.4+ --- arch/mips/kernel/vdso.c | 20 ++++++++++++++++++++ 1 file changed, 20 insertions(+)

diff --git a/arch/mips/kernel/vdso.c b/arch/mips/kernel/vdso.c index 5649a9e429e0..aca06b18c43e 100644 --- a/arch/mips/kernel/vdso.c +++ b/arch/mips/kernel/vdso.c @@ -14,12 +14,14 @@ #include <linux/init.h> #include <linux/ioport.h> #include <linux/irqchip/mips-gic.h> +#include <linux/kernel.h> #include <linux/mm.h> #include <linux/sched.h> #include <linux/slab.h> #include <linux/timekeeper_internal.h>

#include <asm/abi.h> +#include <asm/page.h> #include <asm/vdso.h>

/* Kernel-provided data used by the VDSO. */ @@ -118,12 +120,30 @@ int arch_setup_additional_pages(struct linux_binprm *bprm, int uses_interp) vvar_size = gic_size + PAGE_SIZE; size = vvar_size + image->size;

+ /* + * Find a region that's large enough for us to perform the + * colour-matching alignment below. + */ + if (cpu_has_dc_aliases) + size += shm_align_mask + 1; + base = get_unmapped_area(NULL, 0, size, 0, 0); if (IS_ERR_VALUE(base)) { ret = base; goto out; }

+ /* + * If we suffer from dcache aliasing, ensure that the VDSO data page + * mapping is coloured the same as the kernel's mapping of that memory. + * This ensures that when the kernel updates the VDSO data userland + * will observe it without requiring cache invalidations. + */ + if (cpu_has_dc_aliases) { + base = __ALIGN_MASK(base, shm_align_mask); + base += ((unsigned long)&vdso_data - gic_size) & shm_align_mask; + } + data_addr = base + gic_size; vdso_addr = data_addr + PAGE_SIZE;