[PATCH AUTOSEL 5.10 30/34] 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>
---

Now I understand the issue! Let me analyze the problem: **Analysis:**
**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. `ap_is_valid_header()` accepts a `struct acpi_table_header
*table` parameter 2. `struct acpi_table_header` has a `signature` field
that is only **4 bytes** (`char signature[ACPI_NAMESEG_SIZE]` where
`ACPI_NAMESEG_SIZE = 4`) 3. The original `ACPI_VALIDATE_RSDP_SIG(a)`
macro calls `strncmp(a, ACPI_SIG_RSDP, 8)` - comparing **8 bytes** 4.
When `table->signature` (4 bytes) is passed to this macro, `strncmp`
reads **beyond the bounds** of the signature field, causing a buffer
overread ### The Fix: The new macro implementation: ```c #define
ACPI_VALIDATE_RSDP_SIG(a) (!strncmp (ACPI_CAST_PTR (char, (a)),
ACPI_SIG_RSDP, (sizeof(a) < 8) ? ACPI_NAMESEG_SIZE : 8)) ``` This
intelligently chooses the comparison length: - If `sizeof(a) < 8`, it
uses `ACPI_NAMESEG_SIZE` (4 bytes) - Otherwise, it uses the full 8 bytes
for legitimate RSDP signatures ### Why This Should Be Backported: 1.
**Security Issue**: This is a buffer overread that could potentially
lead to information disclosure or memory corruption 2. **Minimal Risk**:
The change is a small, surgical fix to a macro with clear logic 3.
**Wide Impact**: The macro is used in multiple places including: -
Kernel ACPI subsystem (`drivers/acpi/acpica/tbprint.c`, `tbxfroot.c`) -
Boot code (`arch/x86/boot/compressed/acpi.c`) - ACPI tools (userspace
tools) 4. **Compiler Warnings**: This likely fixes `-Wstringop-overread`
compiler warnings, improving code quality 5. **No Functional Change**:
The fix maintains the same intended behavior while preventing the
overread 6. **Follows Stable Rules**: This is exactly the type of
important bugfix that stable trees are meant to include ### Similar
Commit Precedent: Similar commit #3 (Status: YES) shows that ACPI
compilation warning fixes have been backported before, and this is a
more serious issue (actual buffer overread vs. just truncation
warnings). The fix addresses a real memory safety issue in a fundamental
ACPI macro used throughout the kernel, making it an excellent candidate
for stable backporting.

 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 7334037624c5c..a2bf54fb946a0 100644
--- a/include/acpi/actypes.h
+++ b/include/acpi/actypes.h
@@ -524,7 +524,7 @@ 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