common: relocate fdt_blob in global_data for FDTSRC_EMBED case
Patch resolves two kind of bugs, one of which is vulnerability related
to KASLR.
=== Issue briefly ===
Working with FDT (via non-relocated gd::fdt_blob) from inside bootm
command may lead to the reading the garbage instead of FDT nodes. And
this can result in various side-effects depending on DTS nodes, being
parsed during bootm.
But below is my specific story how I faced with this issue due to
MESON_RNG probing failure.
=== Bugs description ===
1) Bug is revealed on:
* configuration below
* U-boot 2024.10 -
f919c3a889f ("Prepare v2024.10")
It seems, the following patch is a trigger:
ea955eea4f ("fdt: automatically add /chosen/kaslr-seed if DM_RNG is enabled")
Generally, CONFIG_OF_EMBED=y & CONFIG_RNG_MESON=y are the most
valuable ones for reproducing the issue.
```
CONFIG_ARCH_FIXUP_FDT_MEMORY=y
CONFIG_CMD_FDT=y
CONFIG_DEFAULT_FDT_FILE=""
CONFIG_FDT_64BIT=y
CONFIG_OF_BOARD_SETUP=y
CONFIG_OF_CONTROL=y
CONFIG_OF_EMBED=y
CONFIG_OF_LIBFDT_ASSUME_MASK=0x0
CONFIG_OF_LIBFDT_OVERLAY=y
CONFIG_OF_LIBFDT=y
CONFIG_OF_LIST="meson-axg-our-device-name"
CONFIG_OF_REAL=y
CONFIG_OF_TRANSLATE=y
CONFIG_SUPPORT_OF_CONTROL=y
CONFIG_SYS_FDT_PAD=0x3000
CONFIG_TOOLS_OF_LIBFDT=y
CONFIG_DM_RNG=y
CONFIG_RNG_MESON=y
```
2) Due to CONFIG_OF_EMBED, the DTS is embedded into U-boot ELF and
accessible via __dtb_dt_begin symbol.
On early boot stage (board_f.c) the fdtdec_setup() is called only
once before U-boot's relocation into top of RAM. fdtdec_setup()
initializes gd::fdt_blob for FDTSRC_EMBED case:
```
gd->fdt_blob = dtb_dt_embedded();
gd->fdt_src = FDTSRC_EMBED;
```
3) Then reloc_fdt() is called in board_f.c
But due to CONFIG_OF_EMBED=y the reloc_fdt() does not update
gd::fdt_blob value (strictly speaking, it is impossible for
CONFIG_OF_EMBED=y, because U-boot ELF has not been relocated yet
at this moment).
As a result after relocation we get fdt_blob, pointing to DTS address
before relocation:
```
# bdinfo
<...>
relocaddr = 0x000000000fedf000
reloc off = 0x000000000eedf000
<...>
fdt_blob = 0x010ce6c0 << points to __dtb_dt_begin before relocation
new_fdt = 0x0000000000000000 << empty erroneously
fdt_size = 0x0000000000000000 << zero erroneously
```
4) During bootm command (according to our ITS-config file) the Linux
is loaded into 0x01080000 (which is very close to fdt_blob addr
0x010ce6c0).
```
## Loading kernel from FIT Image at
04000000 ...
Trying 'kernel' kernel subimage
<...>
Load Address: 0x01080000
```
So Linux image overwrites the gd::fdt_blob memory location
in RAM (0x010ce6c0).
5) Issue:
Hence any manipulation with DTS (say, via FDT API) inside
implementation of bootm command leads to accessing the fdt_blob area
with garbage, that can lead to two situations:
5.1) Abort.
Call to fdt_off_dt_struct() from fdt_next_tag() :: fdt_offset_ptr()::
fdt_offset_ptr_() returns with garbage, that leads to tagp value
being out of RAM top addr (256 Mb in our board), causing the abort:
```
Boot cmd: bootm 0x4000000#boot_evt1
bootm_run_states()
<...>
image_setup_libfdt()
fdt_chosen()
fdt_kaslrseed()
uclass_get_device()
uclass_get_device_tail()
device_probe()
device_of_to_plat()
meson_rng_of_to_plat()
clk_get_by_name_optional()
clk_get_by_name()
clk_get_by_name_nodev()
ofnode_stringlist_search()
fdt_stringlist_search()
fdt_getprop()
fdt_get_property_namelen_()
fdt_first_property_offset()
fdt_check_node_offset_()
fdt_next_tag():
```
tagp = fdt_offset_ptr(fdt, offset, FDT_TAGSIZE);
```
fdt_next_tag() tagp:0x22890766
fdt_next_tag() ram_top:0x10000000 (tagp OUT of RAM)
"Synchronous Abort" handler, esr 0x96000010, far 0x22890766
elr:
000000000108be24 lr :
000000000108be24 (reloc)
elr:
000000000ff6fe24 lr :
000000000ff6fe24
x0 :
0000000000000041 x1 :
0000000000000000
x2 :
000000000ff3b57c x3 :
0000000000000012
x4 :
000000000ded2ad5 x5 :
0000000000000020
x6 :
00000000ffffffe8 x7 :
000000000ded2f40
x8 :
00000000ffffffd8 x9 :
000000000000000d
x10:
0000000000000006 x11:
000000000001869f
x12:
000000000fffffff x13:
000000000fffffff
x14:
0000000000000000 x15:
000000000ded2abb
x16:
000000000ff3b080 x17:
0000000000000001
x18:
000000000ded3dc0 x19:
0000000022890766
x20:
00000000010cb0f0 x21:
00000000000015e4
x22:
000000000ff8f4d8 x23:
000000000000000b
x24:
000000000ded2fbc x25:
000000000ffe2000
x22:
000000000ff8f4d8 x23:
000000000000000b
x24:
000000000ded2fbc x25:
000000000ffe2000
x26:
000000000ffe2000 x27:
000000000000000b
x28:
000000000ff9cf2d x29:
000000000ded2f40
Code:
aa1603e1 91197484 52801742 94004de8 (
b9400276)
```
5.2) Vulnerability situation "KASLR is disabled".
Almost the same as in (5.1), but 2 situations happen (depending on
the value of garbage):
* call to fdt_offset_ptr_() :: fdt_off_dt_struct(fdt)
returns not so big garbage, leading to tagp, being inside RAM.
* or calculations of absoffset inside fdt_offset_ptr() leads to
failure of the one of if() conditions with NULL as retval.
Result is fdt_next_tag() interprets the tagp as FDT_END. And we are
returning from our callstack via functions' error paths, leading to
"No RNG device" and "KASLR disabled due to lack of seed":
```
fdt_kaslrseed()
uclass_get_device()
<...>
device_probe()
device_of_to_plat()
meson_rng_of_to_plat()
clk_get_by_name()
clk_get_by_name_nodev()
<...>
fdt_stringlist_search()
fdt_getprop()
fdt_get_property_namelen_()
fdt_first_property_offset()
fdt_check_node_offset_()
fdt_next_tag():
```
tagp = fdt_offset_ptr(fdt, offset, FDT_TAGSIZE);
```
fdt_next_tag() tagp:
0000000001890677
fdt_next_tag() ram_top:0x10000000 (tagp is inside RAM)
uclass_get_device_tail():486 device_probe() ret:-22
No RNG device
Starting kernel ...
[ 0.000000] Linux version 6.9.12
[ 0.000000] KASLR disabled due to lack of seed
```
Signed-off-by: Evgeny Bachinin <EABachinin@salutedevices.com>
projects / u-boot.git / commit