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Re: [PATCH] linux-user: Ensure mmap_min_addr is non-zero
From: |
John Paul Adrian Glaubitz |
Subject: |
Re: [PATCH] linux-user: Ensure mmap_min_addr is non-zero |
Date: |
Mon, 27 Jul 2020 13:09:32 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 |
Hi!
On 7/24/20 11:23 PM, Richard Henderson wrote:
> When the chroot does not have /proc mounted, we can read neither
> /proc/sys/vm/mmap_min_addr nor /proc/sys/maps.
>
> The enforcement of mmap_min_addr in the host kernel is done by
> the security module, and so does not apply to processes owned
> by root. Which leads pgd_find_hole_fallback to succeed in probing
> a reservation at address 0. Which confuses pgb_reserved_va to
> believe that guest_base has not actually been initialized.
>
> We don't actually want NULL addresses to become accessible, so
> make sure that mmap_min_addr is initialized with a non-zero value.
>
> Buglink: https://bugs.launchpad.net/qemu/+bug/1888728
> Reported-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
> Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
> ---
> linux-user/main.c | 16 ++++++++++++++--
> 1 file changed, 14 insertions(+), 2 deletions(-)
>
> diff --git a/linux-user/main.c b/linux-user/main.c
> index 3597e99bb1..75c9785157 100644
> --- a/linux-user/main.c
> +++ b/linux-user/main.c
> @@ -758,14 +758,26 @@ int main(int argc, char **argv, char **envp)
>
> if ((fp = fopen("/proc/sys/vm/mmap_min_addr", "r")) != NULL) {
> unsigned long tmp;
> - if (fscanf(fp, "%lu", &tmp) == 1) {
> + if (fscanf(fp, "%lu", &tmp) == 1 && tmp != 0) {
> mmap_min_addr = tmp;
> - qemu_log_mask(CPU_LOG_PAGE, "host mmap_min_addr=0x%lx\n",
> mmap_min_addr);
> + qemu_log_mask(CPU_LOG_PAGE, "host mmap_min_addr=0x%lx\n",
> + mmap_min_addr);
> }
> fclose(fp);
> }
> }
>
> + /*
> + * We prefer to not make NULL pointers accessible to QEMU.
> + * If we're in a chroot with no /proc, fall back to 1 page.
> + */
> + if (mmap_min_addr == 0) {
> + mmap_min_addr = qemu_host_page_size;
> + qemu_log_mask(CPU_LOG_PAGE,
> + "host mmap_min_addr=0x%lx (fallback)\n",
> + mmap_min_addr);
> + }
> +
> /*
> * Prepare copy of argv vector for target.
> */
>
This fixes the problem for me, therefore:
Tested-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
Adrian
--
.''`. John Paul Adrian Glaubitz
: :' : Debian Developer - glaubitz@debian.org
`. `' Freie Universitaet Berlin - glaubitz@physik.fu-berlin.de
`- GPG: 62FF 8A75 84E0 2956 9546 0006 7426 3B37 F5B5 F913