Re: [RFC PATCH v1 18/25] unwind_user/eh_frame: Add support for DWARF expressions

[Jens Remus <jremus@linux.ibm.com>]

On 8/18/2026 5:13 PM, sashiko-bot@kernel.org wrote:

> This commit enables architectures to handle DWARF expressions in DWARF CFI
> instructions. It limits the maximum expression length to a reasonable size
> and provides inline stubs for architectures to override.

>> diff --git a/include/linux/unwind_user_eh_frame_types.h b/include/linux/unwind_user_eh_frame_types.h

>> +struct eh_frame_reg_state {
>> +	/* CFA recovery rule */
>> +	enum eh_frame_cfa_rule cfa_rule;
>> +	unsigned long cfa_regnum;
>> +	long cfa_offset;
> 
> [Severity: High]
> What ensures cfa_offset is initialized when processing CFI programs?
> 
> When struct eh_frame_cfi_context ctx is allocated on the kernel stack in
> __do_cfi_program(), its state.cfa_rule is initialized, but state.cfa_offset
> is not.
> 
> If a user-supplied .eh_frame begins with DW_CFA_def_cfa_register, it sets
> ctx.state.cfa_rule = CFA_REG_OFFSET but leaves cfa_offset untouched.

Fixed in "unwind_user/eh_frame: Add support for reading .eh_frame
section".

> 
> When __find_frame_row() executes, it verifies cfa_rule == CFA_REG_OFFSET
> and unconditionally reads the uninitialized kernel stack memory. Could this
> cause the unwinder to use unpredictable values for the CFA offset?

>> diff --git a/kernel/unwind/eh_frame.c b/kernel/unwind/eh_frame.c

>> +		case DW_CFA_def_cfa_expression: {
>> +			unsigned long expr_len;
>> +			char expr[EH_FRAME_MAX_EXPRESSION_LENGTH];
>> +
>> +			ret = read_uleb128(&cur, end, &expr_len);
>> +			if (ret)
>> +				return ret;
>> +
>> +			if (cur + expr_len > end)

			if (cur + expr_len < cur || cur + expr_len > end)

> 
> [Severity: High]
> Can the pointer arithmetic wrap around if expr_len is maliciously large?
> 
> Since expr_len is parsed from untrusted user input as an unsigned long, a
> large value could overflow cur + expr_len and wrap to a small value,
> bypassing this check against end.
> 
> If this occurs, the parsing loop would advance cur by the overflowed
> expr_len, moving cur backwards. Would this lead to an infinite parsing loop
> and a hard lockup?

Valid.  Improved overflow check.

> 
>> +				return -EINVAL;
>> +
>> +			if (expr_len > sizeof(expr))
>> +				return -EOPNOTSUPP;
>> +			unsafe_copy_from_user(&expr, (void __user *)cur, expr_len, Efault);
> 
> [Severity: Critical]
> Might unsafe_copy_from_user() improperly toggle SMAP during expression
> parsing?
> 
> Because this parsing runs inside a scoped_user_read_access_size block,
> calling unsafe_copy_from_user() on x86 defaults to __copy_from_user(),
> which executes its own stac() and clac() instructions.
> 
> The inner clac() instruction clears the AC flag prematurely, ending the
> UACCESS region while the outer block assumes it is still open.
> 
> When the loop continues to parse the next instruction using
> UNSAFE_GET_USER_INC, it will attempt a direct user memory read with the AC
> flag cleared. Will this result in a fatal page fault?

False.  IIUC config option HAVE_UNWIND_USER_EH_FRAME cannot be manually
enabled.  Therefore this code cannot be enabled on any architecture,
including x86.

unsafe_copy_from_user() is added for x86 by subsequent patch
"[RFC PATCH v1 19/25] x86/uaccess: Add unsafe_copy_from_user()
implementation".

> 
>> +			ret = eh_frame_do_def_cfa_expression(expr, expr_len, target_ip, &ctx->state);
> 
> [Severity: Medium]
> Is it safe to call an architecture hook inside a UACCESS region?
> 
> If an architecture overrides eh_frame_do_def_cfa_expression() with an
> out-of-line external function, it will trigger objtool's 'call to XXX()
> with UACCESS enabled' warning.
> 
> Executing arbitrary functions with the AC flag set is strictly forbidden
> on x86 to prevent SMAP bypasses.

Steven, Josh, what are your thoughts?

> 
>> +			if (ret)
>> +				return ret;
>> +			cur += expr_len;
>> +			break;
>> +		}
Thanks and regards,
Jens
-- 
Jens Remus
Linux on Z Development (D3303)
jremus@de.ibm.com / jremus@linux.ibm.com

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