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Re: [PATCH] linux-user/elfload: fix address calculation in fallback scen

From: Vincent Fazio
Subject: Re: [PATCH] linux-user/elfload: fix address calculation in fallback scenario
Date: Tue, 9 Mar 2021 15:04:57 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0

Pinging per Laurent.

On 2/13/21 3:48 PM, Laurent Vivier wrote:
Le 31/01/2021 à 07:19, Vincent Fazio a écrit :
From: Vincent Fazio <vfazio@gmail.com>

Previously, guest_loaddr was not taken into account when returning an
address from pgb_find_hole when /proc/self/maps was unavailable which
caused an improper guest_base address to be calculated.

This could cause a SIGSEGV later in load_elf_image -> target_mmap for
ET_EXEC type images since the mmap MAP_FIXED flag is specified which
could clobber existing mappings at the address returnd by g2h().

   mmap(0xd87000, 16846912, PROT_NONE, 
   munmap(0xd87000, 16846912)              = 0
   write(2, "Locating guest address space @ 0"..., 40Locating guest address 
space @ 0xd87000) = 40
   mmap(0x1187000, 16850944, PROT_NONE, 
   --- SIGSEGV {si_signo=SIGSEGV, si_code=SEGV_ACCERR, si_addr=0x2188310} ---
   +++ killed by SIGSEGV +++

Now, pgd_find_hole accounts for guest_loaddr in this scenario.

Fixes: ad592e37dfcc ("linux-user: provide fallback pgd_find_hole for bare 
Signed-off-by: Vincent Fazio <vfazio@gmail.com>
  linux-user/elfload.c | 3 ++-
  1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/linux-user/elfload.c b/linux-user/elfload.c
index 8d425f9ed0..6d606b9442 100644
--- a/linux-user/elfload.c
+++ b/linux-user/elfload.c
@@ -2242,7 +2242,8 @@ static uintptr_t pgb_find_hole(uintptr_t guest_loaddr, 
uintptr_t guest_size,
      brk = (uintptr_t)sbrk(0);
if (!maps) {
-        return pgd_find_hole_fallback(guest_size, brk, align, offset);
+        ret = pgd_find_hole_fallback(guest_size, brk, align, offset);
+        return ret - guest_loaddr;
/* The first hole is before the first map entry. */

Reviewed-by: Laurent Vivier <laurent@vivier.eu>

CC: Alex

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