[LTP] [PATCH v2] ldt.h: Add workaround for x86_64

[Andrea Cervesato]

Hi!

On 5/12/25 12:05, Ricardo B. Marlière via ltp wrote:
> From: Ricardo B. Marlière <rbm@suse.com>
>
> The commit be0aaca2f742 ("syscalls/modify_ldt: Add lapi/ldt.h") left behind
> an important factor of modify_ldt(): the kernel intentionally casts the
> return value to unsigned int. This was handled in
> testcases/cve/cve-2015-3290.c but was removed. Add it back to the relevant
> file.
>
> Reported-by: Martin Doucha <mdoucha@suse.cz>
> Signed-off-by: Ricardo B. Marlière <rbm@suse.com>
> ---
> Changes in v2:
> - Added TBROK for any ret != 0 in modify_ldt call in cve-2015-3290.c
> - Link to v1: https://lore.kernel.org/r/20250507-fixes-modify_ldt-v1-1-70a2694cfddc@suse.com
> ---
>   include/lapi/ldt.h            | 22 +++++++++++++++++++++-
>   testcases/cve/cve-2015-3290.c |  8 +++++++-
>   2 files changed, 28 insertions(+), 2 deletions(-)
>
> diff --git a/include/lapi/ldt.h b/include/lapi/ldt.h
> index 6b5a2d59cb41bfc24eb5ac26c3d47d49fb8ff78f..173321dd9ac34ba87eff0eee960635f30d878991 100644
> --- a/include/lapi/ldt.h
> +++ b/include/lapi/ldt.h
> @@ -31,7 +31,27 @@ struct user_desc {
>   static inline int modify_ldt(int func, const struct user_desc *ptr,
>   			     unsigned long bytecount)
>   {
> -	return tst_syscall(__NR_modify_ldt, func, ptr, bytecount);
> +	long rval;
> +
> +	errno = 0;
> +	rval = tst_syscall(__NR_modify_ldt, func, ptr, bytecount);
> +
> +#ifdef __x86_64__
> +	/*
> +	 * The kernel intentionally casts modify_ldt() return value
> +	 * to unsigned int to prevent sign extension to 64 bits. This may
> +	 * result in syscall() returning the value as is instead of setting
> +	 * errno and returning -1.
> +	 */
> +	if (rval > 0 && (int)rval < 0) {
> +		tst_res(TINFO,
> +			"WARNING: Libc mishandled modify_ldt() return value");
> +		errno = -(int)errno;
> +		rval = -1;
> +	}
> +#endif /* __x86_64__ */
This code is correct, but check comment below..
> +
> +	return rval;
>   }
>   
>   static inline int safe_modify_ldt(const char *file, const int lineno, int func,
> diff --git a/testcases/cve/cve-2015-3290.c b/testcases/cve/cve-2015-3290.c
> index 8ec1d53bbb5a9f3e7761d39855d34f593e118a28..e70742acc87c39088953e02f16146b7b58a75fd1 100644
> --- a/testcases/cve/cve-2015-3290.c
> +++ b/testcases/cve/cve-2015-3290.c
> @@ -197,7 +197,13 @@ static void set_ldt(void)
>   		.useable	 = 0
>   	};
>   
> -	SAFE_MODIFY_LDT(1, &data_desc, sizeof(data_desc));
> +	TEST(modify_ldt(1, &data_desc, sizeof(data_desc)));
> +	if (TST_RET == -1 && TST_ERR == EINVAL) {
> +		tst_brk(TCONF | TTERRNO,
> +			"modify_ldt: 16-bit data segments are probably disabled");
> +	} else if (TST_RET != 0) {
> +		tst_brk(TBROK | TTERRNO, "modify_ldt");
> +	}
What about handling this in safe_modify_ldt, so all tests using 
SAFE_MODIFY_LDT() will use it?
>   }
>   
>   static void try_corrupt_stack(unsigned short *orig_ss)
>
> ---
> base-commit: b070a5692e035ec12c3d3c7a7e9e97c270fd4d7d
> change-id: 20250507-fixes-modify_ldt-ebcfdd2a7d30
>
> Best regards,
- Andrea