Re: [PATCH kvm-unit-tests] fw_cfg: avoid index out of bounds

[Nadav Amit <nadav.amit@gmail.com>]

> 
> On Jul 30, 2020, at 2:58 PM, Paolo Bonzini <pbonzini@redhat.com> wrote:
> 
> clang compilation fails with
> 
> lib/x86/fwcfg.c:32:3: error: array index 17 is past the end of the array (which contains 16 elements) [-Werror,-Warray-bounds]
>                fw_override[FW_CFG_MAX_RAM] = atol(str) * 1024 * 1024;
> 
> The reason is that FW_CFG_MAX_RAM does not exist in the fw-cfg spec and was
> added for bare metal support.  Fix the size of the array and rename FW_CFG_MAX_ENTRY
> to FW_CFG_NUM_ENTRIES, so that it is clear that it must be one plus the
> highest valid entry.
> 
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
> lib/x86/fwcfg.c | 6 +++---
> lib/x86/fwcfg.h | 5 ++++-
> 2 files changed, 7 insertions(+), 4 deletions(-)
> 
> diff --git a/lib/x86/fwcfg.c b/lib/x86/fwcfg.c
> index c2aaf5a..1734afb 100644
> --- a/lib/x86/fwcfg.c
> +++ b/lib/x86/fwcfg.c
> @@ -4,7 +4,7 @@
> 
> static struct spinlock lock;
> 
> -static long fw_override[FW_CFG_MAX_ENTRY];
> +static long fw_override[FW_CFG_NUM_ENTRIES];
> static bool fw_override_done;
> 
> bool no_test_device;
> @@ -15,7 +15,7 @@ static void read_cfg_override(void)
> 	int i;
> 
> 	/* Initialize to negative value that would be considered as invalid */
> -	for (i = 0; i < FW_CFG_MAX_ENTRY; i++)
> +	for (i = 0; i < FW_CFG_NUM_ENTRIES; i++)
> 		fw_override[i] = -1;
> 
> 	if ((str = getenv("NR_CPUS")))
> @@ -44,7 +44,7 @@ static uint64_t fwcfg_get_u(uint16_t index, int bytes)
>     if (!fw_override_done)
>         read_cfg_override();
> 
> -    if (index < FW_CFG_MAX_ENTRY && fw_override[index] >= 0)
> +    if (index < FW_CFG_NUM_ENTRIES && fw_override[index] >= 0)
> 	    return fw_override[index];
> 
>     spin_lock(&lock);
> diff --git a/lib/x86/fwcfg.h b/lib/x86/fwcfg.h
> index 64d4c6e..ac4257e 100644
> --- a/lib/x86/fwcfg.h
> +++ b/lib/x86/fwcfg.h
> @@ -20,9 +20,12 @@
> #define FW_CFG_NUMA             0x0d
> #define FW_CFG_BOOT_MENU        0x0e
> #define FW_CFG_MAX_CPUS         0x0f
> -#define FW_CFG_MAX_ENTRY        0x10
> +
> +/* Dummy entries used when running on bare metal */
> #define FW_CFG_MAX_RAM		0x11
> 
> +#define FW_CFG_NUM_ENTRIES      (FW_CFG_MAX_RAM + 1)
> +
> #define FW_CFG_WRITE_CHANNEL    0x4000
> #define FW_CFG_ARCH_LOCAL       0x8000
> #define FW_CFG_ENTRY_MASK       ~(FW_CFG_WRITE_CHANNEL | FW_CFG_ARCH_LOCAL)
> — 
> 2.26.2

For the record: I did send a patch more than two weeks ago to fix this
problem (that I created).