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linux-user: Ensure mmap_min_addr is non-zero

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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>
Tested-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
Acked-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20200724212314.545877-1-richard.henderson@linaro.org>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>
This commit is contained in:
Richard Henderson 2020-07-24 14:23:14 -07:00 committed by Laurent Vivier
parent 9303ecb658
commit c9f8066697
1 changed files with 14 additions and 2 deletions
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16
linux-user/main.c
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@ -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.
*/