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

[Laurent Vivier]

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, 
> MAP_PRIVATE|MAP_ANONYMOUS|MAP_NORESERVE|0x100000, -1, 0) = 0xd87000
>   munmap(0xd87000, 16846912)              = 0
>   write(2, "Locating guest address space @ 0"..., 40Locating guest address 
> space @ 0xd87000) = 40
>   mmap(0x1187000, 16850944, PROT_NONE, 
> MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS|MAP_NORESERVE, -1, 0) = 0x1187000
>   --- 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 
> chroots")
> 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