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spl: fit: bound the external data size before reading it

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load_simple_fit() loads an image stored as external data by reading
it from the boot device with a transfer sized from the FIT data-size
property. That property is listed in exc_prop[] in image-fit-sig.c,
so it is excluded from the configuration signature and stays under
the control of anyone able to modify the boot medium even when
CONFIG_SPL_FIT_SIGNATURE is enabled. The read happens before
fit_image_verify_with_data() checks the image hash, so an inflated
data-size overruns the destination before the corruption can be
detected. The device-tree overlay path is the sharpest case, because
there the destination is a fixed CONFIG_SPL_LOAD_FIT_APPLY_OVERLAY_BUF_SZ
heap buffer.

Pass the size of the destination into load_simple_fit() and reject
an image whose data does not fit before the read is issued. The
check is done in two places: an early bail on len > max_size, then a
bail on the block-aligned size > max_size. The size check is the
mathematically binding one because size is len rounded up to the
device block length. The early bail exists so that
get_aligned_image_size() never runs on a hostile len, where its int
arithmetic would invoke signed-integer overflow.

For the overlay path the bound is exact: the caller passes the size
of its temporary buffer. For the firmware, loadables, FDT and FPGA
call sites the destination is wherever the load_addr field points,
with no defined upper limit at the call site. Those callers pass
CONFIG_SYS_BOOTM_LEN as a conservative ceiling, matching the same
limit spl_parse_legacy_validate() already applies to legacy images.
It is not a tight bound on the actual capacity at the destination,
just a cap that rejects implausibly-sized data.

Signed-off-by: Aristo Chen <aristo.chen@canonical.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
This commit is contained in:
Aristo Chen
2026-07-21 13:50:54 -06:00
committed by Tom Rini
parent f5f06b10e1
commit 8f71d7170f
1 changed files with 45 additions and 6 deletions
Download Patch File Download Diff File Expand all files Collapse all files
common/spl/spl_fit.c
+45 -6
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@@ -204,6 +204,9 @@ static int get_aligned_image_size(struct spl_load_info *info, int data_size,
* If the FIT node does not contain a "load" (address) property,
* the image gets loaded to the address pointed to by the
* load_addr member in this struct, if load_addr is not 0
* @max_size: maximum number of bytes that may be written to the
* destination; an image whose data exceeds this is rejected
* before it is read from the device
*
* Return: 0 on success, -EBADSLT if this image is not the correct phase
* (for CONFIG_BOOTMETH_VBE_SIMPLE_FW), or another negative error number on
@@ -211,7 +214,7 @@ static int get_aligned_image_size(struct spl_load_info *info, int data_size,
*/
static int load_simple_fit(struct spl_load_info *info, ulong fit_offset,
const struct spl_fit_info *ctx, int node,
struct spl_image_info *image_info)
struct spl_image_info *image_info, ulong max_size)
{
int offset;
size_t length;
@@ -291,6 +294,23 @@ static int load_simple_fit(struct spl_load_info *info, ulong fit_offset,
return 0;
}
/*
* data-size is excluded from the configuration signature (it
* is in exc_prop[] in image-fit-sig.c), so it stays attacker
* controlled even after fit_config_verify() succeeds. The
* image hash is only verified after the device read below, so
* an oversized value has to be rejected here.
*
* Bail out before get_aligned_image_size() runs on a hostile
* len: that helper does its arithmetic in int and would
* invoke signed-integer overflow on a value close to or above
* INT_MAX. The block-aligned check further down is the
* mathematically binding one, since size is len rounded up to
* the device block length.
*/
if ((ulong)len > max_size)
goto too_big;
if (spl_decompression_enabled() &&
(image_comp == IH_COMP_GZIP || image_comp == IH_COMP_LZMA))
src_ptr = map_sysmem(ALIGN(CONFIG_SYS_LOAD_ADDR, ARCH_DMA_MINALIGN), len);
@@ -302,6 +322,15 @@ static int load_simple_fit(struct spl_load_info *info, ulong fit_offset,
size = get_aligned_image_size(info, length, offset);
read_offset = fit_offset + get_aligned_image_offset(info,
offset);
/*
* info->read() transfers the block-aligned size into the
* destination, so this is the bound that actually matters;
* len was rejected above only to keep this computation safe.
*/
if (size > max_size)
goto too_big;
log_debug("reading from offset %x / %lx size %lx to %p: ",
offset, read_offset, size, src_ptr);
@@ -372,6 +401,11 @@ static int load_simple_fit(struct spl_load_info *info, ulong fit_offset,
upl_add_image(fit, node, load_addr, length);
return 0;
too_big:
printf("%s: FIT image too large (data-size %u, max %lu)\n",
__func__, (u32)len, max_size);
return -EFBIG;
}
static bool os_takes_devicetree(uint8_t os)
@@ -427,7 +461,8 @@ static int spl_fit_append_fdt(struct spl_image_info *spl_image,
spl_image->fdt_addr = map_sysmem(image_info.load_addr, size);
memcpy(spl_image->fdt_addr, gd->fdt_blob, size);
} else {
ret = load_simple_fit(info, offset, ctx, node, &image_info);
ret = load_simple_fit(info, offset, ctx, node, &image_info,
CONFIG_SYS_BOOTM_LEN);
if (ret < 0)
return ret;
@@ -479,7 +514,8 @@ static int spl_fit_append_fdt(struct spl_image_info *spl_image,
}
image_info.load_addr = (ulong)tmpbuffer;
ret = load_simple_fit(info, offset, ctx, node,
&image_info);
&image_info,
CONFIG_SPL_LOAD_FIT_APPLY_OVERLAY_BUF_SZ);
if (ret == -EBADSLT)
continue;
else if (ret < 0)
@@ -687,7 +723,8 @@ static int spl_fit_load_fpga(struct spl_fit_info *ctx,
warn_deprecated("'fpga' property in config node. Use 'loadables'");
/* Load the image and set up the fpga_image structure */
ret = load_simple_fit(info, offset, ctx, node, &fpga_image);
ret = load_simple_fit(info, offset, ctx, node, &fpga_image,
CONFIG_SYS_BOOTM_LEN);
if (ret) {
printf("%s: Cannot load the FPGA: %i\n", __func__, ret);
return ret;
@@ -849,7 +886,8 @@ int spl_load_simple_fit(struct spl_image_info *spl_image,
}
/* Load the image and set up the spl_image structure */
ret = load_simple_fit(info, offset, &ctx, node, spl_image);
ret = load_simple_fit(info, offset, &ctx, node, spl_image,
CONFIG_SYS_BOOTM_LEN);
if (ret)
return ret;
@@ -890,7 +928,8 @@ int spl_load_simple_fit(struct spl_image_info *spl_image,
continue;
image_info.load_addr = 0;
ret = load_simple_fit(info, offset, &ctx, node, &image_info);
ret = load_simple_fit(info, offset, &ctx, node, &image_info,
CONFIG_SYS_BOOTM_LEN);
if (ret < 0 && ret != -EBADSLT) {
printf("%s: can't load image loadables index %d (ret = %d)\n",
__func__, index, ret);