Re: [PATCH 02/14] x86/hw_breakpoint: Prevent data breakpoints on direct GDT

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 29/05/2020 22:27, Peter Zijlstra wrote:
> From: Lai Jiangshan <laijs@linux.alibaba.com>
>
> A data breakpoint on the GDT is terrifying and should be avoided.
> The GDT on CPU entry area is already protected. The direct GDT
> should be also protected, although it is seldom used and only
> used for short time.

While I agree with the sentiment...

>
> Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> Link: https://lkml.kernel.org/r/20200526014221.2119-3-laijs@linux.alibaba.com
> ---
>  arch/x86/kernel/hw_breakpoint.c |   30 ++++++++++++++++++++++--------
>  1 file changed, 22 insertions(+), 8 deletions(-)
>
> --- a/arch/x86/kernel/hw_breakpoint.c
> +++ b/arch/x86/kernel/hw_breakpoint.c
> @@ -32,6 +32,7 @@
>  #include <asm/processor.h>
>  #include <asm/debugreg.h>
>  #include <asm/user.h>
> +#include <asm/desc.h>
>  
>  /* Per cpu debug control register value */
>  DEFINE_PER_CPU(unsigned long, cpu_dr7);
> @@ -237,13 +238,26 @@ static inline bool within_area(unsigned
>  }
>  
>  /*
> - * Checks whether the range from addr to end, inclusive, overlaps the CPU
> - * entry area range.
> + * Checks whether the range from addr to end, inclusive, overlaps the fixed
> + * mapped CPU entry area range or other ranges used for CPU entry.
>   */
> -static inline bool within_cpu_entry_area(unsigned long addr, unsigned long end)
> +static inline bool within_cpu_entry(unsigned long addr, unsigned long end)
>  {
> -	return within_area(addr, end, CPU_ENTRY_AREA_BASE,
> -			   CPU_ENTRY_AREA_TOTAL_SIZE);
> +	int cpu;
> +
> +	/* CPU entry erea is always used for CPU entry */
> +	if (within_area(addr, end, CPU_ENTRY_AREA_BASE,
> +			CPU_ENTRY_AREA_TOTAL_SIZE))
> +		return true;
> +
> +	for_each_possible_cpu(cpu) {
> +		/* The original rw GDT is being used after load_direct_gdt() */
> +		if (within_area(addr, end, (unsigned long)get_cpu_gdt_rw(cpu),
> +				GDT_SIZE))

... why the O(n) loop over the system?

It is only GDTs which might ever be active on this local CPU(/thread)
which are a problem, because the breakpoint registers are similarly local.

Nothing is going to go wrong If I put a breakpoint on someone else's
live GDT, because they wont interact in the "fun" ways we're trying to
avoid.

~Andrew