From nobody Sat Feb 7 17:48:58 2026 Received: from read.uberspace.de (read.uberspace.de [185.26.156.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 6F3B7187FFC for ; Mon, 12 Aug 2024 17:45:25 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=185.26.156.133 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1723484727; cv=none; b=WoAcYV4eMmefXv7cAhQXyF0PQEGPATMlUsHTS3wxg8zvdoeEMRMbpgnyT2M433f4g8RaYV+YBwX6K7RrDLHGJLwL4HVU1kfaapaBSO38MXQpVH5mQTg/FdF0qA+MCZrEnBDndMCVMpdWbkMPqykkKJQulxaw0KYusrN1v7IUrHQ= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1723484727; c=relaxed/simple; bh=8RV+PP8sYQd1tMJEbzZNDRlelpeGUX71LiVKe+Q840c=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=qXgyZTndtVMLmj61vFirl+IVJ18UU8BiTns0S81sFcmjiG3Az2VLMCcs+1MrxJpSilwTmOYhQRjwDRILqdTB6eqfyU571ri5SrLSBCR44dQPEYZN3ZWhznGpMtc5fKsd1/FQ4CZ80JybEbngiCMCsd8RMkAWdSHC2edkTyeBQjE= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=florommel.de; spf=pass smtp.mailfrom=florommel.de; dkim=pass (4096-bit key) header.d=florommel.de header.i=@florommel.de header.b=SmC2XXRl; arc=none smtp.client-ip=185.26.156.133 Authentication-Results: smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=florommel.de Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=florommel.de Authentication-Results: smtp.subspace.kernel.org; dkim=pass (4096-bit key) header.d=florommel.de header.i=@florommel.de header.b="SmC2XXRl" Received: (qmail 19575 invoked by uid 990); 12 Aug 2024 17:45:21 -0000 Authentication-Results: read.uberspace.de; auth=pass (plain) Received: from unknown (HELO unkown) (::1) by read.uberspace.de (Haraka/3.0.1) with ESMTPSA; Mon, 12 Aug 2024 19:45:21 +0200 From: Florian Rommel To: Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , "H . Peter Anvin" , Jason Wessel , Daniel Thompson , Douglas Anderson , Lorena Kretzschmar , Stefan Saecherl , Peter Zijlstra , Christophe JAILLET , Randy Dunlap , Masami Hiramatsu , Andrew Morton , Christophe Leroy , Geert Uytterhoeven , kgdb-bugreport@lists.sourceforge.net, x86@kernel.org, linux-kernel@vger.kernel.org Cc: Florian Rommel Subject: [PATCH v2 1/2] x86/kgdb: convert early breakpoints to poke breakpoints Date: Mon, 12 Aug 2024 19:43:36 +0200 Message-ID: <20240812174338.363838-2-mail@florommel.de> X-Mailer: git-send-email 2.46.0 In-Reply-To: <20240812174338.363838-1-mail@florommel.de> References: <20240812174338.363838-1-mail@florommel.de> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable X-Rspamd-Bar: ---- X-Rspamd-Report: REPLY(-4) SUSPICIOUS_RECIPS(1.5) MID_CONTAINS_FROM(1) MIME_GOOD(-0.1) BAYES_HAM(-2.999751) R_MISSING_CHARSET(0.5) X-Rspamd-Score: -4.099751 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=florommel.de; s=uberspace; h=from:to:cc:subject:date; bh=8RV+PP8sYQd1tMJEbzZNDRlelpeGUX71LiVKe+Q840c=; b=SmC2XXRlE1cZrd+LZ9TdwYeiZCDUanumd9YD8C9amP0Zrn2hUOEX1NH1jT+2PxcTp4P8ptLpCt Ij4dsfkHLO5W3+MUp0P8+Tg1OLEE2FIIasA7R7XH/AmcYIMBarmk8CYSHqRUi8pctsWBSgugpkOE KMKVZiiZ9jXG2lNRvHoiRIDq8ACeaioisxL8x/EeXcRxsuKOGafRopZlLEdjOdc5KPQo0Cufu3xv Xo5M5W6iGmTPQ2o62QqgLICRdtw8ymFzDYt1dLwwxG/4X3ZQIq5aTaBi0ujCjK8XZwkqE/eEn+VS k950OJknORvBbwpeiwrX4vU5BKFYgxSwiLh3hgsyZKSiTFbH8DcKPLPiEeAqDxyS6xyGpwKrsUoD IuPWoSqkPw2smC2itKUm7YT/OqRwcs+hHGRw6ss5TzTpyepZuaOKBUf0JSxpKWysP/eb2vlhWHow 20yQa4lDOhTDvGmczb9RiwRB/QIIGhl8koccWH4eM6D6E64g4SWEaMfSGJckuzRvcZhBUP6ZDaJj fqaNgE7CThokVoAnZkAYk8ixoAYbyMft3qQ4rAHAOq3B6uH27zgBfKIpMRMiksfFCAvVa192IYZ4 Q4dyai0NlHP9Gpqw30Izf6y50jedikoqoo4TV4Z7QScwuZGF9vemGQqwkTprpk42KL2opjh0Ukk3 U= Content-Type: text/plain; charset="utf-8" On x86, after booting, the kernel text is read-only. Then, KGDB has to use the text_poke mechanism to install software breakpoints. KGDB uses a special (x86-specific) breakpoint type for these kinds of breakpoints (BP_POKE_BREAKPOINT). When removing a breakpoint, KGDB always adheres to the breakpoint's original installment method, which is determined by its type. Before this fix, early (non-"poke") breakpoints could not be removed after the kernel text was set as read-only since the original code patching mechanism was no longer allowed to remove the breakpoints. Eventually, this even caused the kernel to hang (loop between int3 instruction and the function kgdb_skipexception). With this patch, we convert early breakpoints to "poke" breakpoints after the kernel text has been made read-only. This makes them removable later. Signed-off-by: Florian Rommel --- v2: Add missing stub implementation for kgdb_after_mark_readonly A patch for this problem has already been proposed by Stefan Saecherl and Lorena Kretzschmar [1]. Their solution is different from the one I suggest here (it fixes the problem on removal, not "in advance"). Unfortunately, Lorena and Stefan's patch has not been accepted / the conversation has fallen asleep. One point of criticism concerned possible problems with reused init code pages. This should not be a problem with my patch. [1] https://lore.kernel.org/all/20201214141314.5717-1-stefan.saecherl@fau.d= e/ arch/x86/kernel/kgdb.c | 14 ++++++++++++++ include/linux/kgdb.h | 4 ++++ init/main.c | 1 + kernel/debug/debug_core.c | 7 ++++++- 4 files changed, 25 insertions(+), 1 deletion(-) diff --git a/arch/x86/kernel/kgdb.c b/arch/x86/kernel/kgdb.c index 9c9faa1634fb..64c332151af7 100644 --- a/arch/x86/kernel/kgdb.c +++ b/arch/x86/kernel/kgdb.c @@ -623,6 +623,20 @@ int kgdb_arch_init(void) return retval; } =20 +void kgdb_after_mark_readonly(void) +{ + int i; + + /* Convert all breakpoints in rodata to BP_POKE_BREAKPOINT. */ + for (i =3D 0; i < KGDB_MAX_BREAKPOINTS; i++) { + if (kgdb_break[i].state !=3D BP_UNDEFINED && + kgdb_break[i].type =3D=3D BP_BREAKPOINT && + is_kernel_text(kgdb_break[i].bpt_addr)) { + kgdb_break[i].type =3D BP_POKE_BREAKPOINT; + } + } +} + static void kgdb_hw_overflow_handler(struct perf_event *event, struct perf_sample_data *data, struct pt_regs *regs) { diff --git a/include/linux/kgdb.h b/include/linux/kgdb.h index 76e891ee9e37..903bf833dca1 100644 --- a/include/linux/kgdb.h +++ b/include/linux/kgdb.h @@ -98,6 +98,8 @@ extern int dbg_set_reg(int regno, void *mem, struct pt_re= gs *regs); # define KGDB_MAX_BREAKPOINTS 1000 #endif =20 +extern struct kgdb_bkpt kgdb_break[KGDB_MAX_BREAKPOINTS]; + #define KGDB_HW_BREAKPOINT 1 =20 /* @@ -360,11 +362,13 @@ extern bool dbg_is_early; extern void __init dbg_late_init(void); extern void kgdb_panic(const char *msg); extern void kgdb_free_init_mem(void); +extern void kgdb_after_mark_readonly(void); #else /* ! CONFIG_KGDB */ #define in_dbg_master() (0) #define dbg_late_init() static inline void kgdb_panic(const char *msg) {} static inline void kgdb_free_init_mem(void) { } +static inline void kgdb_after_mark_readonly(void) {} static inline int kgdb_nmicallback(int cpu, void *regs) { return 1; } #endif /* ! CONFIG_KGDB */ #endif /* _KGDB_H_ */ diff --git a/init/main.c b/init/main.c index 206acdde51f5..33b6e092fed3 100644 --- a/init/main.c +++ b/init/main.c @@ -1441,6 +1441,7 @@ static void mark_readonly(void) mark_rodata_ro(); debug_checkwx(); rodata_test(); + kgdb_after_mark_readonly(); } else if (IS_ENABLED(CONFIG_STRICT_KERNEL_RWX)) { pr_info("Kernel memory protection disabled.\n"); } else if (IS_ENABLED(CONFIG_ARCH_HAS_STRICT_KERNEL_RWX)) { diff --git a/kernel/debug/debug_core.c b/kernel/debug/debug_core.c index ce1bb2301c06..9dd6b69f1679 100644 --- a/kernel/debug/debug_core.c +++ b/kernel/debug/debug_core.c @@ -98,7 +98,7 @@ module_param(kgdbreboot, int, 0644); * Holds information about breakpoints in a kernel. These breakpoints are * added and removed by gdb. */ -static struct kgdb_bkpt kgdb_break[KGDB_MAX_BREAKPOINTS] =3D { +struct kgdb_bkpt kgdb_break[KGDB_MAX_BREAKPOINTS] =3D { [0 ... KGDB_MAX_BREAKPOINTS-1] =3D { .state =3D BP_UNDEFINED } }; =20 @@ -452,6 +452,11 @@ void kgdb_free_init_mem(void) } } =20 +void __weak kgdb_after_mark_readonly(void) +{ + /* Weak implementation, may be overridden by arch code */ +} + #ifdef CONFIG_KGDB_KDB void kdb_dump_stack_on_cpu(int cpu) { --=20 2.46.0 From nobody Sat Feb 7 17:48:58 2026 Received: from read.uberspace.de (read.uberspace.de [185.26.156.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 93980191 for ; Mon, 12 Aug 2024 17:45:28 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=185.26.156.133 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1723484730; cv=none; b=a/ZMZiwth7dTIsn/tgQtLmYaElQhHV5xEktLXosgVRVZECRH/4WUDCuBj6JBswMQ2WqRenuhTU9fSHdvBABqchoCX6IlB75tQGuQFbn7W6r9WQjUsSjpGf4v3MNmswljfRrO2GQhVBEQ/R2PWSa5GjsbDF1G7T1RNuxGa+iPmoI= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1723484730; c=relaxed/simple; bh=83ni0oxgvU7wO3TGJK4AznLbgJf+G+hKoJ+YstNEvTc=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=J5xEeXss/Tum3ibH2Wo6yRJowGqAYrwyoPM2i1YTKxNw+UVaQZJJIRqrn0vsJXREDxRRZ35aRyI+XODgcyNDrOx6+zeO/Lk6nKlAJUk4YuvzEuvFJiLd+EMvAtuzaqLflerVT3wfatHws+hJ7IJaM8mm+q17nGYU3xceBOFW50Y= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=florommel.de; spf=pass smtp.mailfrom=florommel.de; dkim=pass (4096-bit key) header.d=florommel.de header.i=@florommel.de header.b=rm9lbJ5M; arc=none smtp.client-ip=185.26.156.133 Authentication-Results: smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=florommel.de Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=florommel.de Authentication-Results: smtp.subspace.kernel.org; dkim=pass (4096-bit key) header.d=florommel.de header.i=@florommel.de header.b="rm9lbJ5M" Received: (qmail 19654 invoked by uid 990); 12 Aug 2024 17:45:25 -0000 Authentication-Results: read.uberspace.de; auth=pass (plain) Received: from unknown (HELO unkown) (::1) by read.uberspace.de (Haraka/3.0.1) with ESMTPSA; Mon, 12 Aug 2024 19:45:25 +0200 From: Florian Rommel To: Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , "H . Peter Anvin" , Jason Wessel , Daniel Thompson , Douglas Anderson , Lorena Kretzschmar , Stefan Saecherl , Peter Zijlstra , Christophe JAILLET , Randy Dunlap , Masami Hiramatsu , Andrew Morton , Christophe Leroy , Geert Uytterhoeven , kgdb-bugreport@lists.sourceforge.net, x86@kernel.org, linux-kernel@vger.kernel.org Cc: Florian Rommel Subject: [PATCH v2 2/2] x86/kgdb: fix hang on failed breakpoint removal Date: Mon, 12 Aug 2024 19:43:37 +0200 Message-ID: <20240812174338.363838-3-mail@florommel.de> X-Mailer: git-send-email 2.46.0 In-Reply-To: <20240812174338.363838-1-mail@florommel.de> References: <20240812174338.363838-1-mail@florommel.de> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable X-Rspamd-Bar: ---- X-Rspamd-Report: REPLY(-4) SUSPICIOUS_RECIPS(1.5) MID_CONTAINS_FROM(1) MIME_GOOD(-0.1) BAYES_HAM(-2.956524) R_MISSING_CHARSET(0.5) X-Rspamd-Score: -4.056524 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=florommel.de; s=uberspace; h=from:to:cc:subject:date; bh=83ni0oxgvU7wO3TGJK4AznLbgJf+G+hKoJ+YstNEvTc=; b=rm9lbJ5McaCPuNfKtT2SiDAwQtqhDensizQBTK8IZxXKNzhsvDR/nBh63pG+uyFHOdMHIJknS3 Sijwsb/BAYu7ijxQVG7ny486hiZZFsvPMtPWSjDTH20PXkowK47mBvh2ZSnw3mEZFqR1Xenu5DIW YV7FhD9fS+opIE/IGc7CQjOi4pY5WrHtuYL9E1kbuhcXgKP39MwfZDHfhuA6GHlZWOFU8ulHvlYn cWF1w2ONqBpQNpr11MHYS/pz1O9Wnqgt2ZhcxLIjr1BxcRqJ1CDQzFimXvY8+fISusDyTjN6HSOd iCqaD1UFuxLe1xAgPxHAs2N990HvSELBJ+4UQjISDNdWwK/l0ecohrPmLXXlCWiOCx0FO8bIQFYM 8xpnqsxLqkgwVpQVAlBlPcctS7bRTn1AvWRtjxH2At1BdHZJ7zrcfBn0dG8BuaGnCPxpGPGcvsvT gtAdilX9gBD51XnWPin18glsYKWKJy+VPefuZFz6g5Oj66i1UeOkGQFRt/pIEG4x/ah5PEfhrsfP WsIpMLaG8h9v+Qmr1UShV9bWcE340x396QjAgBVXsmNGHPboOYWBGFTH/bCXUAH4ahb/LQ0kg7+c CUUPaN5KNeZ9TmnbMgW94riLE9iAVoWOCffORZaNRX4oacrNxlKp3aRmUAft6XM/bOmsx1zutUpI I= Content-Type: text/plain; charset="utf-8" On x86, occasionally, the removal of a breakpoint (i.e., removal of the int3 instruction) fails because the text_mutex is taken by another CPU (mainly due to the static_key mechanism, I think). The function kgdb_skipexception catches exceptions from these spurious int3 instructions, bails out of KGDB, and continues execution from the previous PC address. However, this led to an endless loop between the int3 instruction and kgdb_skipexception since the int3 instruction (being still present) triggered again. This effectively caused the system to hang. With this patch, we try to remove the concerned spurious int3 instruction in kgdb_skipexception before continuing execution. This may take a few attempts until the concurrent holders of the text_mutex have released it, but eventually succeeds and the kernel can continue. Signed-off-by: Florian Rommel --- arch/x86/kernel/kgdb.c | 24 ++++++++++++++++++++++++ 1 file changed, 24 insertions(+) diff --git a/arch/x86/kernel/kgdb.c b/arch/x86/kernel/kgdb.c index 64c332151af7..585a7a72af74 100644 --- a/arch/x86/kernel/kgdb.c +++ b/arch/x86/kernel/kgdb.c @@ -723,7 +723,31 @@ void kgdb_arch_exit(void) int kgdb_skipexception(int exception, struct pt_regs *regs) { if (exception =3D=3D 3 && kgdb_isremovedbreak(regs->ip - 1)) { + struct kgdb_bkpt *bpt; + int i, error; + regs->ip -=3D 1; + + /* + * Try to remove the spurious int3 instruction. + * These int3s can result from failed breakpoint removals + * in kgdb_arch_remove_breakpoint. + */ + for (bpt =3D NULL, i =3D 0; i < KGDB_MAX_BREAKPOINTS; i++) { + if (kgdb_break[i].bpt_addr =3D=3D regs->ip && + kgdb_break[i].state =3D=3D BP_REMOVED && + (kgdb_break[i].type =3D=3D BP_BREAKPOINT || + kgdb_break[i].type =3D=3D BP_POKE_BREAKPOINT)) { + bpt =3D &kgdb_break[i]; + break; + } + } + if (!bpt) + return 1; + error =3D kgdb_arch_remove_breakpoint(bpt); + if (error) + pr_err("skipexception: breakpoint remove failed: %lx\n", + bpt->bpt_addr); return 1; } return 0; --=20 2.46.0