[PATCH AUTOSEL 6.6 07/18] ACPICA: Avoid sequence overread in call to strncmp()

[Sasha Levin <sashal@kernel.org>]

From: Ahmed Salem <x0rw3ll@gmail.com>

[ Upstream commit 64b9dfd0776e9c38d733094859a09f13282ce6f8 ]

ACPICA commit 8b83a8d88dfec59ea147fad35fc6deea8859c58c

ap_get_table_length() checks if tables are valid by
calling ap_is_valid_header(). The latter then calls
ACPI_VALIDATE_RSDP_SIG(Table->Signature).

ap_is_valid_header() accepts struct acpi_table_header as an argument, so
the signature size is always fixed to 4 bytes.

The problem is when the string comparison is between ACPI-defined table
signature and ACPI_SIG_RSDP. Common ACPI table header specifies the
Signature field to be 4 bytes long[1], with the exception of the RSDP
structure whose signature is 8 bytes long "RSD PTR " (including the
trailing blank character)[2]. Calling strncmp(sig, rsdp_sig, 8) would
then result in a sequence overread[3] as sig would be smaller (4 bytes)
than the specified bound (8 bytes).

As a workaround, pass the bound conditionally based on the size of the
signature being passed.

Link: https://uefi.org/specs/ACPI/6.5_A/05_ACPI_Software_Programming_Model.html#system-description-table-header [1]
Link: https://uefi.org/specs/ACPI/6.5_A/05_ACPI_Software_Programming_Model.html#root-system-description-pointer-rsdp-structure [2]
Link: https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html#index-Wstringop-overread [3]
Link: https://github.com/acpica/acpica/commit/8b83a8d8
Signed-off-by: Ahmed Salem <x0rw3ll@gmail.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Link: https://patch.msgid.link/2248233.Mh6RI2rZIc@rjwysocki.net
Signed-off-by: Sasha Levin <sashal@kernel.org>
---

**YES** This commit should be backported to stable kernel trees. ##
Detailed Analysis: ### The Problem The commit fixes a **buffer overread
vulnerability** in the `ACPI_VALIDATE_RSDP_SIG` macro. The issue occurs
when: 1. **Context**: The macro `ACPI_VALIDATE_RSDP_SIG(a)` calls
`strncmp(ACPI_CAST_PTR(char, (a)), ACPI_SIG_RSDP, 8)` 2. **Problem**:
When `a` points to a standard ACPI table header (4 bytes signature), but
the comparison tries to read 8 bytes for RSDP validation ("RSD PTR "),
it reads 4 bytes beyond the allocated signature field 3. **Impact**:
This causes a sequence overread that can be detected by tools like GCC's
`-Wstringop-overread` and security-focused architectures like
CHERI/Morello ### The Fix The fix changes line 530 in
`include/acpi/actypes.h`: ```c // Before: #define
ACPI_VALIDATE_RSDP_SIG(a) (!strncmp (ACPI_CAST_PTR (char, (a)),
ACPI_SIG_RSDP, 8)) // After: #define ACPI_VALIDATE_RSDP_SIG(a) (!strncmp
(ACPI_CAST_PTR (char, (a)), ACPI_SIG_RSDP, (sizeof(a) < 8) ?
ACPI_NAMESEG_SIZE : 8)) ``` This conditionally limits the comparison to
`ACPI_NAMESEG_SIZE` (4 bytes) when the source is smaller than 8 bytes,
preventing the overread. ### Why This Should Be Backported: 1.
**Security Fix**: This addresses a buffer overread that could be
exploited or cause crashes on security-hardened systems 2. **Small &
Contained**: The fix is a single line change to a macro definition with
minimal risk 3. **Widespread Usage**: The macro is used in multiple
critical ACPI code paths: - `drivers/acpi/acpica/tbprint.c` (kernel ACPI
table printing) - `tools/power/acpi/tools/acpidump/apdump.c` (ACPI
debugging tools) - Various other ACPI validation functions 4. **No
Architectural Changes**: This doesn't change functionality, just
prevents unsafe memory access 5. **Compiler Warning Fix**: Resolves
build warnings with newer GCC versions using `-Wstringop-overread` ###
Comparison with Historical Commits: - **Similar to Commit #3 (YES)**:
Like the "disable -Wstringop-truncation" commit, this fixes compiler
warnings related to string operations, but this one actually fixes the
underlying safety issue rather than just suppressing warnings - **Unlike
Commits #1,#2,#4,#5 (NO)**: Those were either pure code cleanup,
debugging improvements, or infrastructure changes without security
implications ### Risk Assessment: - **Minimal Risk**: The fix only
affects boundary condition handling in string comparison - **No
Functional Impact**: Normal RSDP validation still works correctly for
valid 8-byte signatures - **Improves Safety**: Prevents potential memory
safety violations on hardened systems This is exactly the type of small,
safe, security-focused bugfix that stable trees are designed for.

 include/acpi/actypes.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/acpi/actypes.h b/include/acpi/actypes.h
index 85c2dcf2b7048..0a24200852415 100644
--- a/include/acpi/actypes.h
+++ b/include/acpi/actypes.h
@@ -527,7 +527,7 @@ typedef u64 acpi_integer;
 
 /* Support for the special RSDP signature (8 characters) */
 
-#define ACPI_VALIDATE_RSDP_SIG(a)       (!strncmp (ACPI_CAST_PTR (char, (a)), ACPI_SIG_RSDP, 8))
+#define ACPI_VALIDATE_RSDP_SIG(a)       (!strncmp (ACPI_CAST_PTR (char, (a)), ACPI_SIG_RSDP, (sizeof(a) < 8) ? ACPI_NAMESEG_SIZE : 8))
 #define ACPI_MAKE_RSDP_SIG(dest)        (memcpy (ACPI_CAST_PTR (char, (dest)), ACPI_SIG_RSDP, 8))
 
 /* Support for OEMx signature (x can be any character) */
-- 
2.39.5